SYNAPTA  VIVIPARA 


A  CONTRIBUTION  TO  THE  MORPHOLOGY  OF  ECHINODERMS 


A  DISSERTATION 

V 

ACCEPTED    FOK    THE    DEGREE    OF    DOCTOR    OF    PHILOSOPHY    BY    THE    BOARD    OF 
UNIVERSITY   STUDIES   OF   THE   JOHNS    HOPKINS    UNIVERSITY 


1897 


BY  HUBERT  LYMAN  CLARK 


(Reprinted  from  Volume  V,  No.  2,  of  the  Memoirs  of  the  Boston  Society  of  Natural  History, 

and  Volume  IV  of  the  Memoirs  from  the  Biological  Laboratory  of 

the  Johns  Hopkins  ITnietrrity) 


BALTIMORE 
1898 


BIOLOGY 
LIB* 

Q 


3.     SYNAPTA  VIVIPARA  :   A  CONTRIBUTION  TO  THE  MORPHOLOGY  OF  ECHINODERMS. 

BY  HUBERT  LYMAN  CLARK. 

(Read  November  3,  1897.) 


CONTENTS. 

1.  Introductory 53  0.  The  development  of  the  pentactula          .                .  63 

2.  History  and  systematic  position  of  Synapta  vivipara  64  7.  The  development  of  the  adult  Synapta    .                 .  68 

3.  Distribution  and  habits 55  8.  The  anatomy  of  the  adult 75 

4.  Fertilization  and  segmentation  of  the  egg          .        .  58  9.  Conclusion 79 

6.    Gastrulation  and  formation  of  the  hydrocoel  and  10.  Literature .83 

coelomic  vesicles '  .  til  11.  Explanation  of  plates                86 


1.    INTRODUCTORY. 

In  February,  1890,  through  the  kindness  of  Dr.  W.  K.  Brooks,  there  were  placed  in 
my  hands  for  study  a  number  of  specimens  of  a  small  brown  Synapta  from  Jamaica.  In 
their  body-cavities  there  were  numerous  young  ones  in  all  stages  of  development,  so  that 
an  excellent  opportunity  was  offered  for  working  out  the  embryology.  So  interesting 
did  this  prove  that  I  gladly  availed  myself  of  the  privilege  of  spending  the  months  of 
May,  June,  and  July,  1896,  at  Port  Henderson,  Jamaica,  in  the  marine  biological  labora- 
tory of  the  Johns  Hopkins  University.  For  such  a  privilege  I  am  under  the  greatest 
obligations  to  the  authorities  of  that  institution.  During  those  months,  I  studied  the 
segmentation  of  the  egg  and  the  early  stages  of  development  from  living  material  and 
obtained  an  abundance  of  preserved  material  for  further  investigations.  Since  my  return 
I  have  been  engaged  in  a  detailed  study  of  the  development  and  anatomy  of  the  animal 
under  the  direction  of  Dr.  Brooks,  and  it  gives  me  great  pleasure  to  acknowledge  the  debt 
I  am  under  to  him  for  his  suggestions  and  help. 

The  young  were  easily  procured  by  cutting  off  the  heads  of  the  adults  and  thus 
setting  free  the  contents  of  the  body-cavity.  They  could  then  be  killed  as  desired,  but 
young  taken  from  the  body-cavities  of  preserved  adults  were  fully  as  satisfactory  for  all 
purposes.  The  best  results  were  obtained  by  the  use  as  a  fixing  agent  of  "  corrosive- 
acetic  "  (four  parts  corrosive-sublimate  and  one  part  glacial  acetic  acid),  but  excellent 
preservation,  especially  for  the  adults,  was  secured  by  the  use  of  picro-sulphuric  or  picro- 


173213 


54  HUBERT   LYMAN   CLARK  ON 

nitric  acid.  Corrosive-sublimate  alone  gave  poor  results,  while  Perenyi's  fluid  proved 
very  unsatisfactory,  the  material  prepared  with  it  showing  only  fair  preservation,  and 
staining  very  poorly.  Very  good  material  for  the  study  of  the  calcareous  bodies  and 
the  development  of  the  calcareous  ring  was  obtained  by  the  use  of  absolute  alcohol.  In 
staining,  it  was  found  useless  to  try  any  solution  which  contained  less  than  seventy  per 
cent  alcohol,  collapse  and  distortion  of  the  tissues  always  resulting.  Kleinenberg's 
haematoxylin  and  eosin  gave  good  results,  but  acid  borax-carmine  (70%  alcohol)  and 
Lyons  blue  (70%  alcohol)  proved  the  most  satisfactory.  The  early  stages  of  the  larvae 
were  oriented  and  imbedded  in  celloiden  before  imbedding  in  paraffine,  and  thus  kept 
their  shape  very  well.  Although  material  was  so  abundant  and  easily  obtained,  it  is 
impossible  to  tell  the  age  of  the  different  embryos,  for  none  of  the  eggs  obtained 
developed  beyond  the  blastula,  and  later  stages  would  live  outside  of  the  mother  for  only 
a  very  short  time.  Accordingly  it  cannot  be  determined  how  many  days  or  weeks  are 
required  for  the  growth  of  the  pentactula  or  older  stages,  though  there  is  reason  to 
believe  that  growth,  at  least  early  in  life,  is  very  rapid. 

2.   HISTORY  AND  SYSTEMATIC  POSITION  OF  SYNAPTA  VIVIPARA. 

The  Danish  naturalist  Orsted  ('50)  mentioned  the  discovery  in  the  West  Indies  of 
a  new  genus  of  the  Synaptidae,  the  chief  characteristic  of  which  was  its  being  viviparous. 
To  this  genus  he  gave  the  name  Synajjtula,  and  the  type  species  he  called  vivipam.  In 
his  very  brief  and  unsatisfactory  account,  for  which  I  am  indebted  to  Ludwig's  German 
translation  ('81),  he  mentions  the  occurrence  of  the  form  in  "  shallow-water,"  describes 
the  color  as  "greenish,"  and  speaks  of  the  presence  of  "  eyes,"  "skin-glands,"  and 
"anchors."  He  never  published  anything  further  in  regard  to  the  species,  and  later 
writers,  as  Bronn  ('60),  Selenka  ('67),  and  Semper  ('68),  accepted  the  genus  Synaptula 
without  comment,  But  Ludwig  ('81),  in  describing  a  viviparous  Chirodota  (G.  rotifera) 
from  the  coast  of  BraziJ,  suggested  that  it  might  be  the  species  on  which  Orsted  had 
based  his  new  genus.  This  conclusion  was  adopted  by  Lamport  ('85),  and  in  his  work 
we  find  Synaptula  vivipara  given  as  a  synonym  under  hirodota  rotifera.  Theel  ('86) 
placed  Synaptula  vivipara  in  the  list  of  Synaptidae  about  which  little  or  nothing  is  known, 
but  he  suggests  in  agreement  with  Ludwig  ('81)  that  the  stage  of  development  which  the 
eggs  reach  before  leaving  the  mother,  is  not  a  satisfactory  character  upon  which  to  base 
a  new  genus.  Ludwig  ('86)  described  a  small  Synapta,  found  in  floating  seaweed,  west 
of  the  Abrolhos  Reef,  Brazil,  which  he  regarded  as  identical  with  Orsted's  species,  since 
it  contained  young  in  the  body-cavity  and  answered  Orsted's  description,  except  in  color 


SYNAPTA   VIVIPARA.  55 

In  1891,  students  of  the  Johns  Hopkins  University  found  at  Port  Royal,  Jamaica,  a 
viviparous  Synapta,  and,  in  1893,  another  party  from  the  same  institution  brought  back  a 
large  amount  of  preserved  material  of  this  species,  which  they  found  abundantly  about 
Port  Royal.  It  was  this  material  which  came  into  my  hands,  and  after  examining  it  and 
studying  the  living  animal  at  Port  Royal,  I  had  no  difficulty  in  satisfying  myself  that  it, 
as  well  as  Ludwig's  Abrolhos  specimen,  was  indeed  Orsted's  Synaptula  vivipara.  A 
brief  description  to  establish  its  position  in  the  genus  Synapta,  was  published  by  Clark  ('96) , 
and  attention  was  there  called  to  its  similarity  to  S.  picta  Theel  ('96).  This  species  was 
described  from  a  single  specimen  in  the  Challenger  collection,  from  Bermuda,  and  the 
agreement  of  all  its  characters  with  those  of  S.  vivipara  was  very  striking.  Dr.  The"el 
writes  me  that  he  has  specimens  of  a  viviparous  Synapta  from  Bermuda  in  his  hands  at 
the  present  time,  which  agree  with  S.  picta  and  S.  vivipara  so  completely  that  he  has  no 
doubt  that  all  three  are  the  same  species;  but  the  evidence  is  incomplete,  owing  to  the 
absence  of  the  anchors  and  plates  in  his  Bermuda  specimens,  caused  by  the  killing  agent 
used.  Although,  as  we  shall  see,  Synapta  vivipara  differs  in  several  important  particulars 
from  all  other  Synaptas  hitherto  described,  they  are  not  sufficiently  obvious  to  warrant  its 
separation,  under  the  existing  classification  of  the  Synaptidae,  as  a  distinct  genus.  Synap- 
tula must  therefore  become  a  synonym  of  Synapta,  and  the  synonomy  of  S.  vivipara  will 
be  as  follows  :  — 

Synapta  vivipara  (Orst.)  Ludw.  Zool.  Jahrbiicher.     Bd.  2,  p.  28.     1886. 
Synaptula  vivipara  Orsted.  Vid.  med.  fra  d.  nat.  For.  i  Kjobenhavn  for  1849-50,  p.  vii. 
Synaptula  vivipara  Bronn.  Klas.  und  Ord.  d.  Thier.,  Bd.  2,  p.  403.     1860. 
Synaptula  vivipara  Selenka.  Zeit.  fur  wiss.  Zool.,  Bd.  17,  p.  365.     1867. 
Synaptula  vivipara  Semper.  Reis.  in  Arch.  Phil.,  2.  Theil,  1.    Bd.,  p.  24.     1868. 
Chirodota  rotifera  (in  part  only)  Lampert.     Die  Seewalzen.     Wiesbaden.      1885. 
Synaptula   vivipara   Theel.      Report   on    the    Holothurioidea.     "Challenger"    Reports. 

Zool.,  Vol.  14,  p.  32.     1886. 
Synapta  picta  The"el.,  p.  10.     Report  on    the    Holothurioidea.     "Challenger"  Reports. 

Zool.,  Vol.  14,  p.  32.     1886. 

3.   DISTRIBUTION  AND  HABITS. 

Orsted's  specimens  of  Synapta  vivipara,  he  tells  us,  were  from  the  West  Indies, 
and  all  of  the  specimens  I  have  seen,  came  from  Jamaica.  Ludwig's  single  specimen  came 
from  the  Abrolhos  Reef  off  the  coast  of  Brazil  (18°  S.  lat.),  while  all  of  The"el's  specimens 
are  from  Bermuda  (32°  N.  lat.).  We  may  therefore  conclude  that  the  species  is  pretty 


56  HUBERT    LYMAN    CLARK    ON 

widely  distributed  throughout  the  eastern  Atlantic  ocean,  wherever  suitable  conditions 
are  found.  In  Jamaica,  however,  the  species  is  extremely  local  and  was  found  in  only 
one  place,  the  so-called  "  lakes  "  at  Port  Royal.  These  "  lakes  "  are  parts  of  the  harbor 
which  have  been  wholly  or  in  part  cut  off  by  the  growth  of  mangroves,  so  that  they  are 
very  quiet  bodies  of  water,  though  not  at  all  stagnant  or  brackish.  On  the  roots  of  the 
mangroves,  which  hang  down  in  the  water  on  all  sides,  is  an  abundant  growth  of 
vegetable  and  animal  life.  In  some  places,  a  particular  sea-weed,  one  of  the  Florideae, 
crowds  out  all  the  other  Algae.  In  this  weed  Synapta  vivipara  makes  its  home,  and  though 
carefully  looked  for  elsewhere,  it  was  found  in  numbers  only  in  such  situations.  The 
late  Dr.  J.  E.  Humphrey  kindly  identified  this  alga  for  me,  as  Acantliopliora  thierii 
Lamouroux.  This  weed  also  grows  in  large  bunches  on  the  bottom  in  the  shallow  water 
of  the  harbor  just  outside  the  "  lakes,"  and  I  was  told  that  in  1893  the  Synapta  was 
found  in  great  quantities  there,  but  in  1896  it  seemed  to  have  entirely  disappeared 
from  that  place.  At  Montego  Bay,  on  the  northwest  coast  of  Jamaica,  Acanthophora  is 
very  abundant  on  the  mangrove  roots,  but  a  thorough  search  revealed  no  sign  of 
Synaptas  there.  Even  in  the  Port  Royal  "lakes,"  their  distribution  was  very  capricious, 
and  only  certain  favored  masses  of  Acanthophora  contained  them  in  any  numbers.  They 
seem  to  be  quite  social  in  their  habits,  and  usually  if  one  or  two  were  found,  there  would 
be  a  whole  colony  of  them.  They  are  very  sensitive  to  changed  conditions,  and  I  was 
unable  to  keep  them  alive  in  aquaria  more  than  twenty-four  hours.  The  anchors  in  the 
body-wall  are  so  abundant  and  prominent  that  they  cling  very  tenaciously  to  anything 
with  which  they  come  in  contact,  especially  the  hands,  and  it  is  accordingly  no  easy  task 
to  disentangle  them  from  the  sea-weed  without  injury.  They  seem  to  be  able  to  swim 
very  little,  and  it  is  doubtful  if  they  ever  leave  the  bunch  of  sea-weed,  in  which  they 
have  once  settled.  Their  food  consists  largely  of  vegetable  matter,  diatoms  being 
abundant  in  the  stomach,  but  probably  many  small  crustaceans  and  worms  are  also  eaten. 
The  tentacles  are  kept  constantly  in  motion,  and  it  was  very  common  to  find  small 
amphipods  caught  among  them,  but  I  was  unable  to  find  evidence  that  these  crustaceans 
ever  served  as  food.  Semon  ('87)  has  called  attention  to  what  he  considers  a  mimicry 
of  coloration  in  Synapta  inhaerans,  in  relation  to  the  bottom  on  which  it  is  found.  In 
this  connection,  it  is  interesting  to  note  that  the  reddish  and  greenish  brown  shades 
of  S.  vivipara  are  almost  exactly  those  of  the  Acanthophora  in  which  it  lives.  So  close 
is  the  resemblance  that  it  is  very  easy  to  overlook  Synaptas,  even  when  the  sea-weed  is 
in  one's  hand.  Whether  this  coloration  is  actually  protective  or  not  is  doubtful,  for  they 
seem  to  have  few,  if  any,  enemies.  No  internal  parasites  were  observed ;  externally 
however  a  small  brown  calcareous  sponge,  like  Grantia,  was  found  firmly  attached 


SYNAPTA   VIVIPARA.  57 

to  the  skin,  just  behind  the  circle  of  tentacles.  When  placed  in  aquaria,  this 
Synapta  does  not  break  up  by  muscular  contractions,  like  S.  inhaerans,  nor  does  it 
ordinarily  eviscerate  like  many  holothurians,  but  after  crawling  about  restlessly  for  a 
while,  it  stretches  out  on  the  bottom  and  dies,  almost  without  the  contraction  of  a  muscle. 
In  a  few  cases,  evisceration  at  the  mouth  took  place,  when  they  were  thrown  into  a  killing 
agent. 

Breeding  goes  on  all  through  the  spring  and  summer  ;  and  there  is  no  evidence  to 
show  that  it  does  not  go  on  all  the  year  round.  My  earliest  specimens,  collected  April  30 
contained  many  well-developed  young,  and  up  to  the  end  of  July  in  all  the  specimens 
obtained,  young  were  found,  while  Ludwig's  specimen  from  Abrolhos  was  collected  in 
September  and  contained  very  young  embryos.  The  number  of  young  in  the  body- 
cavity  of  a  single  adult  varies  greatly,  depending  more  or  less  on  the  size  of  the  individ- 
ual. Specimens  not  over  a  centimeter  long  may  contain  a  few,  while  some  very  large 
ones  have  scores.  The  largest  number  I  have  found  is  176.  It  is  a  curious  fact  that  the 
young  are  almost  always  in  two  broods  ;  that  is,  a  certain  proportion  of  them  will  all 
have  reached  a  given  age,  say  that  of  the  pentactula,  while  the  remainder  will  be  much 
younger,  say  about  that  of  the  gastrula.  When  the  young  are  very  few,  they  will  all  be 
the  same  age,  while  if  they  are  very  numerous,  they  will  sometimes  show  three  different 
stages.  This  fact  seems  to  indicate  that  the  eggs  ripen  and  pass  into  the  body-cavity  in 
lots  of  from  six  to  a  hundred,  and  that  several  days  elapse  before  another  lot  is  ripened. 
Regarding  the  length  of  time  during  which  the  young  remain  in  the  body-cavity,  it  is 
impossible  to  make  even  an  estimate.  Animals  kept  in  aquaria  frequently  gave  birth  to 
young  only  five  mm.  long,  and  it  was  not  usual  to  find  much  larger  specimens  in  the  adults 
examined,  but  sometimes  a  young  one,  fifteen  or  twenty  mm.  long,  with  all  the  charac- 
ters of  the  adult,  would  be  found  still  inside  its  mother.  Observations  made  on  the  living 
animals  showed  that  birth  occurs  normally  at  the  posterior  end  of  the  body,  apparently 
through  the  anus.  Investigation  showed  that  this  was  accomplished  by  a  rupture  of  the 
body- wall,  which  may  be  through  the  skin  some  little  distance  from  the  anus  (Fig.  29), 
or,  as  seems  to  be  more  generally  the  case,  through  the  wall  of  the  rectum  close  to  the 
point  where  it  joins  the  external  body-wall,  the  young  passing  out  through  the  anal 
opening.  It  may  be  that  the  openings  in  the  wall  of  the  rectum  (Figs.  30  and  31),  to  be 
described  later,  are  concerned  in  the  birth  of  the  young,  but  they  seem  to  be  too  small  to 
be  of  any  service  in  this  connection,  except  possibly  as  starting  points  for  the  rupture  of 
the  rectum  wall.  Under  abnormal  conditions,  I  found  by  experiment,  rupture  of  the 
body-wall  and  consequent  birth  of  the  young  may  occur  at  other  points  than  near  the  anus. 


58  HUBERT   LYMAN   CLARK  ON 

4.   FERTILIZATION  AND  SEGMENTATION  OF  THE  EGG. 

Like  all  other  known  Synaptas,  S.  vivipara  is  hermaphroditic.  Ripe  spermatozoa  and 
ova  are  found  in  the  same  genital  organ  and  even  in  the  same  branch,  and  to  judge  both 
from  living  material  and  preserved  specimens,  even  at  the  same  time;  but  this  latter 
point  could  not  be  proven  since  it  is  impossible  to  determine  simply  by  observation,  when 
the  egg  is  mature.  The  branches  of  the  genital  gland  are  nile-green  in  color  in  the  living 
animal,  while  the  fully  grown  ova  are  brownish  yellow,  and  the  spermatozoa,  when  in 
any  quantity,  appear  to  be  white.  In  shape,  the  latter  are  like  those  figured  by  Jourdan 
('83)  for  Holothuria  tubulosa.  The  mature  egg  (Fig.  1)  is  about  '20(V  in  diameter  and 
before  fertilization  is  not  provided  with  any  membrane.  It  is  full  of  yolk  material  but 
comparatively  transparent,  so  that  the  internal  changes  could  be  watched  up  to  a  late 
stage  of  development.  Artificial  fertilization  proved  unsuccessful,  although  attempted 
several  times.  Whether  self-fertilization  takes  place  or  not  could  not  be  positively 
decided,  but  that  it  is  at  least  very  improbable  seems  clear  from  the  structure  of  the 
genital  organs  and  the  probable  manner  of  egg-laying.  The  genital  gland  of  the  adult 
Synapta  lies  just  above  the  oesophagus,  with  one  or  two  branches  on  each  side  of  the 
dorsal  mesentery  and  with  the  genital  duct  lying  in  that  mesentery  and  opening  to  the 
exterior  close  to  the  base  of  one  of  the  mid-dorsal  tentacles.  A  cross-section  of  one  of 
the  branches  (Fig.  40)  shows  that  it  consists  of  an  external  covering,  the  continuation  of 
the  epithelium,  a  very  scanty  connective-tissue  layer  consisting  only  of  a  few  scattered 
mesenchyme  cells,  and  a  much  folded  germinal  epithelium  surrounding  the  lumen  of  the 
gland.  Of  the  layer  of  circular  muscle  fibers  which  was  found  in  the  genital  gland 
of  S.  digitata  and  S.  inhaerans  by  Quatrefages  ('42),  Baur  ('64),  and  Hamnnn  ('84),  I 
have  found  no  trace  in  S.  vivipara.  The  branches  of  the  gland  contract  vigorously  after 
being  cut  from  the  body,  but  so  far  as  I  could  see,  these  contractions  were  always  longi- 
tudinal. The  germinal  epithelium  is  more  or  less  plainly  made  up  of  two  or  more  layers 
of  cells  of  which  the  external  are  the  larger.  As  will  be  seen  from  figures  39  and  40, 
from  this  external  layer  of  cells  the  ova  arise  and  so  come  to  lie  between  the  germinal 
epithelium  and  the  epithelium  of  the  body-cavity,  and  do  not  pass  into  the  lumen  of  the 
gland  at  all.  This  arrangement  is  quite  the  reverse  of  what  Cuenot  ('91)  has  shown  to 
exist  in  S.  inhaerans,  and  gives  us  a  clue  as  to  how  the  eggs  get  into  the  body-cavity  of 
the  mother.  When  the  eggs  are  mature,  they  press  so  closely  against  the  external 
epithelium  of  the  gland  that  it  bulges  out  sufficiently  to  be  seen  with  the  naked  eye,  in 
the  living  gland.  Sections  show  that  the  epithelium  over  such  ova  is  so  stretched  as  to 
be  thinner  than  elsewhere  (Fig.  39),  and  probably  the  eggs  enter  the  body-cavity  simply 


SYNAPTA   VIVIPARA.  59 

by  the  rupture  of  the  epithelium  at  that  point.  While  this  has  never  been  actually 
observed,  the  theory  is  supported  by  a  further  examination  of  the  genital  gland.  The 
smaller  internal  cells  of  the  germinal  epithelium  give  rise  to  spermatozoa,  which  are 
almost  invariably  to  be  found  in  the  lumen  of  the  gland,  never  on  the  outside  with  the 
ova.  If  we  trace  the  lumen  forward  we  find  it  passes  directly  into  the  lumen  of  the 
genital  duct,  the  internal  walls  of  the  latter  actually  being  formed  by  a  continuation  of 
the  germinal  epithelium,  which  has  become  of  uniform  thickness  and  ciliated  internally 
(Figs.  36  and  37).  The  lumina  of  the  two  or  three  branches  of  the  gland  unite  on 
entering  the  duct  (Fig.  36)  and  pass  through  the  latter  upwards  towards  the  body-wall- 
At  no  point  is  there  any  sign  of  communication  between  the  genital  duct  and  the  space 
external  to  the  germinal  epithelium  in  which  the  ova  lie.  If  we  follow  the  genital  duct 
upwards  to  where  it  comes  in  contact  with  the  body-wall,  we  find  it  does  not  fuse  with 
the  ectoderm  and  open  at  once  to  the  exterior,  but  simply  lies  in  the  connective  tissue 
with  its  end  against  the  ectoderm  (Fig.  38),  and  although  there  are  probably  openings 
through  which  the  spermatozoa  pass  out,  they  are  so  extremely  small  I  have  not  been 
able  to  demonstrate  them  satisfactorily.  In  nearly  all  the  specimens  examined  the 
genital  duct  contained  large  quantities  of  spermatozoa,  but  in  no  case  was  there  any  trace 
of  an  ovum.  That  the  openings  at  the  end  of  the  duct  are  not  directly  continuous  with 
openings  through  the  body-wall  is  indicated  by  the  occurrence  of  spermatozoa,  sometimes 
in  large  quantities,  in  the  connective  tissue  surrounding  the  terminus  of  the  duct  (Fig. 
38).  From  all  these  facts  I  am  convinced  that  the  ova  pass  into  the  body-cavity  by  a  rup- 
ture of  the  peritoneal  epithelium,  while  the  spermatozoa  pass  outward  through  the  genital 
duct  to  the  exterior.  In  Cucumaria  glacialis,  the  only  other  viviparous  holothurian 
concerning  whose  breeding  we  have  any  information,  Mortensen  ('94)  thinks  the  eggs 
are  laid  on  the  bottom  and  taken  up  afterwards  into  the  brood-sacks  of  the  mother.  But 
it  is  manifestly  impossible  that  the  eggs  of  S.  vivipara  could  get  into  the  body-cavity  in 
any  such  way.  The  next  question  that  arises  is,  how  do  the  spermatozoa  reach  the  ova 
inside  the  body-cavity  of  the  mother,  and  the  answer  brings  to  light  another  interesting 
modification  of  structure,  adapted  to  the  viviparous  habit.  Careful  examination  of  the 
rectum  shows  that  through  its  wall  there  are  direct  channels  of  communication  between 
the  body-cavity  and  the  exterior  through  the  anus.  The  wall  of  the  rectum  is  folded 
and  ridged  longitudinally,  and  at  certain  places,  parts  of  these  ridges  have  pushed  out  and 
fused  with  invaginations  from  the  surrounding  coelomic  wall,  forming  distinct  tubes 
connecting  the  interior  of  the  rectum  with  the  interior  of  the  coelom  (Figs. 
30  and  31).  No  trace  of  valves  or  cilia  was  found  in  any  of  these  tubes,  but 
the  passage  of  water  in  and  out  could  be  easily  regulated  by  the  opening  and 


60  HUBERT  LYMAN    CLARK   ON 

closing  of  the  anus.  With  the  anus  open,  each  muscular  contraction  of  the  animal 
would  tend  to  either  force  water  out  or  draw  it  in  through  these  openings,  and  in 
that  way  spermatozoa  could  easily  get  into  the  body-cavity  and  thus  fertilization 
could  take  place  within.  This  will  appear  more  probable  when  it  is  remembered 
that  the  animals  are  very  social,  and  that  the  water  around  a  mangrove  root  on  which 
there  are  hundreds  of  them,  must  contain  countless  spermatozoa.  There  is  also  a 
possibility  that  spermatozoa  enter  the  body-cavity,  through  the  water-pore  and  stone- 
canal,  which,  as  we  shall  see,  remain  open  in  the  adult,  and  also  open  into  the  body-cavity. 
After  fertilization  a  membrane  forms  around  the  egg  and  segmentation  begins.  It 
seems  probable  that  the  extrusion  of  the  polar  bodies  occurs  before  the  formation  of  this 
membrane,  as  Selenka  ('83)  found  no  trace  of  them  in  the  segmenting  eggs  of  &'.  digitata, 
and  I  could  not  find  them  in  any  of  the  eggs  of  S.  vivipara  which  I  examined,  although 
they  were  looked  for  with  special  care.  Segmentation  and  the  formation  of  the  blastula 
occur  in  practically  the  same  manner  as  has  been  so  well  figured  by  Selenka  ('83)  for 
S.  digitata.  The  first  plane  of  division  forms  two  blastomeres  of  equal  size  and  appear- 
ance (Fig.  2).  After  a  resting  period  of  about  twenty  minutes,  the  second  plane  of 
division  occurs  at  right  angles  to  the  first,  giving  rise  to  four  similar  blastomeres  (Fig. 
3).  The  third  plane  is  at  right  angles  to  the  first  two,  and  we  now  have  an  embryo  of 
eight  equal  cells  with  a  segmentation-cavity  between  them  (Fig.  4).  The  sixteen-cell 
stage  (Fig.  5)  soon  follows,  the  division  plane  being  at  right  angles  to  the  preceding. 
The  appearance  of  the  embryo  at  this  stage  is  very  peculiar  and  characteristic,  the  cells 
being  arranged  in  a  band  or  ring  and  the  segmentation-cavity  being  open  at  each  pole. 
Another  plane  of  division,  again  at  right  angles  to  the  preceding,  doubles  the  width  of 
the  band  and  decreases  the  openings  at  the  poles,  but  it  does  not  divide  the  cells  exactly 
equally,  so  that  the  upper-  and  lowermost  rows  are  of  somewhat  smaller  cells  than  the  two 
middle  rows  and  have  a  less  diameter  (Fig.  6  and  7).  The  subsequent  divisions  occur 
with  a  fair  degree  of  regularity  in  alternating  planes,  each  division  decreasing  the 
openings  at  the  poles  until  at  last  they  are  entirely  closed.  This  occurs  when  the  embryo 
consists  of  approximately  256  cells,  and  so  the  blastula  is  formed  (Fig.  8).  The  cells  of 
the  four  equatorial  rows  are  somewhat  larger  than  the  rest,  but  the  difference  is  not  at 
all  noticeable  and  apparently  has  no  significance.  The  divisions  have  followed  on  each 
other  with  great  rapidity  so  that  the  complete  blastula  is  formed  after  about  four  hours, 
while  in  S.  digitata,  according  to  Selenka  ('83),  the  blastula  is  the  result  of  twelve  hours' 
growth.  It  is  for  this  reason,  that  I  am  inclined  to  think  that  the  whole  process  of  devel- 
opment in  S.  vivipara  is  very  rapid. 


SYNAPTA   VIVIPARA.  61 

5.   GASTRULATION  AND  FORMATION  or  THE  HYDROCOEL  AND  COELOMIC  VESICLES. 

Invagination  of  one  of  the  poles  of  the  blastula  soon  forms  the  archenteron  from  the 
blind  end  of  which  the  mesenchyme  cells  now  begin  to  arise,  some  of  the  endodermal 
cells  being  simply  crowded  out  into  the  segmentation-cavity.  I  could  find  no  evidence  at 
all  of  Selenka's  ('83)  two  primitive  mesenchyme  cells,  but  on  the  contrary,  I  found  cells 
all  over  the  archenteron  which  were  in  various  stages  of  passing  into  the  segmentation 
cavity.  On  the  other  hand  in  not  a  single  case  were  there  found  any  of  the  ectoderma- 
cells  forming  mesenchyme,  and  I  feel  no  hesitation  in  affirming  that  the  mesenchymel 
arises  exclusively  from  the  endodermal  cells,  contrary  to  Ludwig's  ('91)  observations  on 
Cucumaria.  The  number  of  mesenchyme  cells  is  comparatively  small,  and  they  never 
become  so  numerous  or  play  so  important  a  part  in  larval  structures  as  they  do  in  S. 
digitata,  judging  from  Semon's  ('88)  account  and  figures.  The  completed  gastrula  (Fig. 
9)  is  covered  with  cilia  which  are  easily  seen  in  the  living  specimens,  but  in  none  of  my 
preserved  material  has  it  been  possible  to  demonstrate  them.  In  a  few  cases  gastrula 
were  found  free  from  the  egg-membrane  and  moving  about  actively  in  the  fluid  of  the 
body-cavity,  but  the  great  majority  are  still  enclosed  in  the  membrane  within  which  they 
rotate  by  means  of  the  cilia.  The  membrane  may  be  retained,  as  preserved  specimens 
show,  until  long  after  the  coelomic  vesicles  are  formed,  and  I  am  inclined  to  believe  that 
there  is  no  definite  time  when  it  is  cast  off,  but  that  it  ruptures  and  is  lost  whenever  the 
larva  has  grown  too  big  for  it.  As  the  archenteron  increases  in  length  it  bends  to  one  side 
and  unites  with  the  wall  of  what  subsequently  becomes  the  dorsal  surface  of  the  larva. 
Its  lumen  breaks  through  the  surface,  and  thus  the  water-pore  is  formed,  as  described  by 
Selenka  ('83)  for  S.  digitata.  Meanwhile  the  gastrula  loses  its  spherical  shape  and 
becomes  more  or  less  elongated  (Fig.  10).  The  cells  at  the  end  opposite  the  blastopore 
are  already  somewhat  different  in  form  from  those  elsewhere  and  make  a  sort  of  plate  of 
thickened  ectoderm  (Fig.  21)  which  may  correspond  to  the  so-called  "neural  plate"  of 
other  echinoderm  larvae.  This  plate,  however,  does  not  lie  exactly  opposite  the  blastopore, 
but  somewhat  toward  the  ventral  side  of  the  larva,  and  as  the  gastrula  increases  in  length, 
it  comes  to  lie  more  and  more  on  that  side.  At  the  same  time,  the  archenteron  continues 
to  increase  in  length  and  grows  forward  and  at  the  same  time  ventralward,  thus  drawing 
away  from  the  water-pore.  In  so  doing,  that  part  of  it  which  grows  forward  pushes  by 
the  part  opening  through  the  water-pore,  on  the  right-hand  side  (looked  at  from  the 
dorsal  surface)  so  that  the  latter  comes  to  lie  on  the  left  side  of  the  larva  (Figs.  Hand 
12).  As  the  archenteron  grows  it  completely  severs  its  connection  with  this  vesicle,  and 


62  HUBERT   LYMAN   CLARK   ON 

pushing  onward  and  downward  against   the  thickened  part  of    the  ectoderm,  becomes 
attached  to  it,  and  the  mouth  breaks  through  at  that  point.     The  mouth  may  be  formed 
before  the  separation  between  the  archenteron  and  the  other  vesicle  is  completed.     Mean- 
while the  latter  increases  in  size,  and  its  walls  become  thinner.    It  grows  backward  toward 
the  blastopore  especially  and  soon  becomes  constricted  and  divided  into  two  vesicle?,  the 
anterior  of  which  is  connected  with  the  exterior  by  the  water-pore,  while  the  posterior  is 
entirely  closed  and  lies  beside  the  posterior  part  of  the  stomach,  if  we  may  so  designate 
the  middle  section  of  the  archenteron  (Fig.  13).     Soon  afterwards  this  posterior  vesicle 
grows  out  laterally  to  the  right,  across  the  dorsal  surface  of  the  larval  hind-gut,  and  forms 
on  the  right-hand  side  a  vesicle  like  itself,  which  soon  becomes  entirely  separate  from  it 
(Fig.  14).     The  anterior  of  these  three  vesicles  is  the  rudimentary  hydrocoel,  while  the 
pair  of  posterior  poiiches  represent  the  right  and  left  coelomic  vesicles.     Their  mode  of 
formation  is  essentially  the  same  as  that  described  by  Selenka  ('83)  for  S.  digitata,  but 
the  relative  size  of  the  various  organs  is  markedly  different,  so  that  the  figures  of  the  same 
stages  show  almost  no  resemblance.     If  figure  15  of  Synapta  vivipara  be  compared  with 
the  auricularia  of  about  the  same  age  which  Semon  ('88,  Plate  6,  Fig.  2)  figures,  this 
difference  will  appear  in  many  ways.     The  rudiments  of  the  two  coelomic  pouches  and 
especially  of  the  hydrocoel  are  very  much  larger  relatively  in  S.  vivipara  than  in  S.  digi- 
tata, while  the  difference  in  the  digestive  tract  is  even  more  noticeable.     In  the  European 
species  there  is  a  well-marked  differentiation  into  fore-,  mid-,  and  hind-gut,  while  in  the 
Jamaican  form  there  is  no  such  distinction  evident.     A  remarkable  difference  is  also  to 
be  seen  in  the  external  form  of  the  larvae.     That  of  S.  vivipara  has  retained  its  elliptical 
shape  and  shows  no  trace  of  ciliated  bands,  while  the  other  has  assumed  the  familiar 
auricularia  form.     Furthermore,  there  is  no  trace  of  calcareous  structures  of  any  kind  in 
S.  vivipara  to  correspond  with  the  plates  at  the  posterior  end  of  auricularia.     In  the 
latter  also,  Semon  ('88)  has  figured  and  described  a  larval  nervous  system,  but  after  care- 
ful investigation  of  this  point,  I  am  convinced  there  is  none  in  the  larva  of  S.  vivipara. 
In  external  form,  the  latter  is  very  regular,  but  with  the  growth  of  the  coelomic  pouches 
the  ectoderm  of  the  dorsal  surface  begins  to  become  thinner  and  more  flattened,  while  that 
around  the  mouth,  especially  posterior  to  it,  shows  a  tendency  to  increase  in  thickness. 
This  difference  between  the  dorsal  and  ventral  surfaces  may  sometimes  be  seen  in  very 
young  larvae  and  is  clearly  shown  in  figure  22.     Before  the  two  coelomic  vesicles  have 
entirely  separated,  the  hydrocoel   has   become  considerably  larger  and    begins  to  grow 
anteriorly  and  toward  the  right,  while  about  the  same  time  five  outgrowths  begin  to  appear 
on  that  side  which  is  furthest  from  the  fore-gut  (Fig.  15).     Soon  after  their  appearance 
five  other  much  smaller  outgrowths'  arise,  one  at  the  right  of  each  of  the  first  five.     The 


SYPATA   VIVIPARA.  63 

first  series  gives  rise  to  the  five  primary  tentacles,  and  the  second  series  corresponds  to 
those  which  in  S.  digitata  give  rise  to  the  radial  water-canals.     In  S.  digitata  however, 
there  is  a  sixth  outgrowth  at  the  extreme  left,  the  rudiment  of  the  Polian  vesicle,  but  in 
S.  vivipara  this  vesicle  is  not  formed  until  after  the  closure  of  the  hydrocoel  ring.     The 
water-canal  enters  this  ring  at  a  point  just  between  the  fourth  primary  tentacle  (counting 
from  before  backwards)  and  the  fourth  secondary  outgrowth  (Fig.  23).     In  this  particu- 
lar my  observations  confirm  Bury  ('89)  in  opposition  to  the  statements  of  Semon  ('88). 
Before  the  outgrowths  of  the  hydrocoel  are  very  evident,  that  part  of  it  which  lies 
dorsally  and  in  immediate  connection  with  the  water-canal  biilges  out,  becomes  thinner- 
walled    than    the    rest    of    the    hydrocoel,   and  gradually  separates  from  it,  but  before 
the    separation    becomes   marked,    the    outgrowth    diminishes    in    size    and    with    the 
increasing   growth    of    the    water-canal    disappears.     This    structure,  I    believe,   is   the 
"anterior    coelom "    which     Bury    ('95)    has    shown    to    exist    in    the    auricularia    of 
S.  digitata.     It   is   very  marked   in   some  specimens    of    S.  vivipara    (Fig.   24),  and  I 
see  no  reason  to  doubt  Bury's  interpretation.     He  does  not  make  very  clear  what  the 
ultimate  fate  of  this  coelom  is  in  S.  digitata,  but  leaves  the  impression  that  it  is  connected 
with  the  subsequent  formation  of  the  madrepore  plate,  as  Ludwig  ('91)  considers  it  to  be 
in  Cucumaria.     In  S.  vivipara  however,  there  is  seldom  any  trace  of    it  left  after  the 
hydrocoel  ring  closes,  and  there  is  no  reason  to  suppose  that  it  has  any  connection  with 
the  much  later  madrepore  openings.     About  the  time  of  the  appearance  of  the  primary 
tentacles,  the  larval  anus,  which  was  the  original  blastopore,  closes  entirely  and   the 
digestive  tract  ends  blindly.     Accordingly  we  now  have  a  regular  elliptical  larva,  about 
a  third  of  a  millimeter  long,  with  the  ventral  ectoderm  much   thicker  than  the   dorsal, 
without  ciliated  bands,  calcareous  particles,  or  nervous  system,  a  mouth  on  the  anterior 
ventral  surface  but  no  anus,  a  well-developed  coelomic  pouch  on  each  side  of  the  digestive 
tract,  and  a  hydrocoel  with  five  primary  tentacles  and  five  secondary  outgrowths,  opening 
to  the  exterior  through   the  dorsal  pore,  by  means  of  an  adradial  water-canal,  upon 
which  may  still  be  seen  the  vestige  of  an  anterior  coelom. 

6.     THE  DEVELOPMENT  OF  THE  PENTACTULA. 

In  such  larvae,  the  ectoderm  of  the  ventral  surface  continues  to  thicken  and  before 
long  is  sharply  set  off  from  the  ectoderm  of  the  rest  of  the  body,  which  consists  of  a 
single  layer  of  cells.     The  thickened  ectoderm  forms  a  circular  field  around  the  mouth 
though  the  latter  does  not  lie  at  its  center,  but  nearer  to  its  anterior  edge.    This  circular 
disc  gradually  sinks  below  the  level  of  the  rest  of  the  ectoderm,  and  the  latter  grows  in 


64  HUBERT  LYMAN  CLARK  ON 

over  it  from  all  sides,  until  the  disc  lies  at  the  bottom  of  a  shallow  cavity,  the  so-called 
"  atrium,"  which  opens  on  the  ventral  surface  through  a  small  poi'e    (Fig.  16).     This 
pore  does  not  lie  directly  over  the  mouth,  but  either  before  or  behind  or  somewhat  to 
one  side.     While  these  external  changes  are  taking  place,  the  hydrocoel  has  continued  its 
growth,  the  anterior  end  passing  across  on  to  the  right  side>of  the  larva  where  it  bends 
downward  and  backward  around  the  oesophagus  to  meet  the  posterior  end  near  the  middle 
line.     The  primary  tentacles  have   increased  considerably  in  size  and  are  growing  up 
around  the  floor  of  the  atrium,  while  the  secondary  outgrowths  also  grow  upward  beside 
them.     No  Polian  vesicle  has  yet  appeared,  and  no  marked  distinctions  between   the 
different  parts  of  the  digestive  tract  are  to  be  seen,  but  the  latter  has  become  very  much 
arched  toward  the  dorsal  surface,  and  the  lumen  of  the  middle  section  is  larger  than  at 
either  end.     A  new  anus  may  have  been  formed  by  the  end  of  the  hind-gut  growing  to 
the  body-wall  on  the  ventral  side  and  an  opening  breaking  through,  but  in  some  cases 
the   definitive   anus  does  not  appear  until  the  pentactula  form  is  nearly  attained.     The 
most    important    changes    have    been    going    on    meanwhile    in    the  coelomic   pouches, 
the    growth  of  which  has    been  very  rapid.      The  left  vesicle   has  grown  more   ante- 
riorly than  the  right,  and  sends  forward  two  finger-like   processes,  one  of  which  passes 
across  the  median  line   into   the   right  side  of  the  larva,  while  the  other  grows  up    to 
the  inner  side  of  the   hydrocoel  ring,  above  the  most  posterior  tentacle,  and  follows  the 
course  of  that  ring  around  the  oesophagus  (Figs.  26-28).     These  anterior  prolongations 
of  the  left  coelom  were  observed  by  Bury  ('89  and  '95)   in  8.  digitata,  but  have  appar- 
ently been  overlooked  by  other  investigators.     The  one  which   passes  on  to  the   right, 
side  of  the  body  fuses  so  soon  with  the  right  coelom  that  I  have  been  unable  to  confirm 
Bury's  further  observations  regarding  it,  and  in  S.  vivipara  its  appearance  might  easily 
be  entirely  overlooked.     But  the   prolongation  which  passes  to  the  hydrocoel  remains 
distinct  through  all  the  later  stages  of  the  larva,  and  its  subsequent  changes  are  easy  to 
trace.     It  soon  loses  its  connection  with  the  left  coelom  and  forms  a  tube,  lying  on  the 
oral  surface  of  the  hydrocoel  ring.     Meanwhile  the  right  and  left  coelomic  pouches  have 
met  and  fused  on   the   ventral  side  of  the  digestive  tract,  while  dorsally  they  are  still 
separate,  the  right  pouch  extending  considerably  further  back  than  the  left.     The  larva 
has  now  reached  the  condition  shown  in  Fig.  16.     In  its  subsequent  growth  the  atrium, 
with  its  thick  ectodermal  floor  and  narrow  opening  to  the  exterior,  moves  to  the  anterior 
end  of  the  larva,  where  it  finally  comes  to  lie,  along  with   the  mouth,  oesophagus,  and 
hydrocoel  ring.     The  latter  has  now  closed,  without  the  formation  of  a  Polian  vesicle, 
somewhat  to  the  left  of  the  middle  line  apparently  (Fig.  25),  but  I  am  not  sure  that  the 
point  of  closure  is  always  on  the  left  side,  for  it  is  by  no  means  easy  to  determine  the 


SYNAPTA  VIVIPARA.  65 

exact  mid-line.  With  the  closure  of  the  hydrocoel,  occurs  the  union  of  the  two  ends  of 
that  coelomic  tube  which  lies  on  its  oral  surface,  so  that  we  now  have  a  circular  sinus 
around  the  oesophagus  just  above  the  water-ring.  This  sinus  is  very  evident  in  young 
Synaptas,  and  Bury's  ('95)  surmise  regarding  its  origin  from  the  left  coelom  is  entirely 
correct.  The  primary  tentacles  are  growing  upward,  not  pushing  the  floor  of  the  atrium 
out  before  them,  as  Semon  ('88)  says,  but  enclosing  the  atrium  within  their  circle  so  that 
the  thickened  sensory  epithelium,  which  they  subsequently  possess,  does  not  arise  as 
Semon  describes.  It  is  clear,  from  Figs.  16  and  17,  that  his  description  could  not  possibly 
apply  to  S.  vimpara.  The  secondary  outgrowths  remain  nearly  unchanged  in  size  and 
show  no  sign  of  bending  backward  to  form  radial  canals.  The  Polian  vessel  is  formed  as 
an  outgrowth  on  the  inner  side  of  the  hydrocoel  ring  in  the  left  dorsal  interradius,  as 
Ludwig  ('23)  found  it  to  be  in  Cucumaria. 

The  digestive  tract  grows  with  greater  rapidity  relatively  than  any  of  the  other 
organs.  Accordingly,  the  oesophagus  pushes  upward  toward  the  atrial  opening,  so  that 
the  thickened  ectodermal  floor  of  the  atrium  lies  surrounding  it,  in  the  form  of  a  poorly 
defined  circumoral  ring.  Continued  growth  pushes  the  oesophagus  against  the  upper 
ectodermal  wall  of  the  atrium,  and  with  that  it  fuses,  leaving  the  circumoral  ring  entirely 
cut  off  from  the  outer  ectoderm  of  the  body  (Figs.  87  and  88).  Meanwhile  the  growth 
of  the  primary  tentacles  has  pushed  this  body-wall  upward  and  outward,  so  that  the 
narrow  slit-like  opening  of  the  atrium  is  gradually  widened  until  it  finally  disappears, 
leaving  the  ectodermal-covered,  anterior  end  of  the  oesophagus  to  form  the  definitive 
mouth,  in  the  center  of  the  circle  of  tentacles.  This  process  is  not  completed  however, 
until  the  pentactula  form  is  fully  assumed.  The  differences  between  this  development 
of  the  mouth  and  circumoral  ring  and  that  given  by  Semon  ('88)  for  S.  digitata  are 
almost  irreconcilable,  but  they  are  all  dependent  on  the  question,  whether  the  five 
primary  tentacles  push  up  through  the  floor  of  the  atrium  or  grow  up  around  it.  The 
latter  is  certainly  the  case  in  S.  vimpara.  While  these  changes  are  taking  place  anteri- 
orly, the  hind-gut  has  increased  in  length  so  that  it  has  not  only  arched  still  more  toward 
the  dorsal  surface  but  has  bent  on  itself  and  formed  a  loop  lying  to  the  left  of  the 
stomach.  The  coelomic  pouches  already  united  and  forming  a  single  cavity  ventrally, 
have  met  in,  or  close  to,  the  mid-dorsal  line  and  by  the  union  of  their  walls  have  formed 
the  dorsal  mesentery.  This  mesentery  follows  pretty  closely  the  curve  of  the  intestine 
and  attaches  it  throughout  its  course,  to  the  body-wall.  The  water-canal  lies  in  the 
anterior  part  of  the  mesentery,  but  whether  that  part  was  formed  in  a  different  manner, 
as  Bury  ('95)  thinks  probable,  it  is  impossible  to  say  from  observations  on  S.  mmpara. 

Meantime  most  important  changes  are  going  on  in  the  circumoral  ring  of  ectoderm 


HE 

UNIVERSITY  , 

OF 

-• 


66  HUBERT  LYMAN  CLARK  ON 

which   we  have  seen    was   formed  from    the  floor   of   the    atrium.     This    ring    is    the 
beginning  of  the  central  nervous  system,  and  from  it  the  tentacle  and  radial  nerves  arise. 
As  the  primary  tentacles  push  upward  past  the  atrium,  they  lie  closely  appressed  to  its 
floor  and  wall.     They  retain  this  position  even  after  the  circumoral  ring  is  pretty  clearly 
denned,  and  before   they  have  grown  much  above  it,  the  radial  nerves  appear  between 
each  pair  of  them  as  outgrowths  of  the  central  ring.     These   outgrowths  pass  directly 
over  the  secondary  outgrowths  of  the  water-vascular  system  and  bend  backwards  to  run 
toward  the  aboral  pole  of  the  body.     Very  soon  after  they  appear  (not  before  them,  as 
Semon  ('88)  describes  for  S.  digitata),  that  part  of  the  circumoral  ring  appressed  to  each 
primary  tentacle  begins  to  grow  upward  with  it  on  its  inner  side,  forming  the   tentacle 
nerves.     As  the  tentacles  continue  to  grow  and  press  the  anterior  body  wall  outward,  the 
ectoderm  which  covers   them  at  the  tip  becomes  noticeably  thickened,  especially  on  the 
outer  side  (Fig.  54),  and  apparently  assumes  a  sensory  function,  probably  in  connection 
with  the  tentacle  nerve.     The  formation  of  the  otocysts  of  Thomson  ('62),  the  "  horor- 
gane  "  of  Baur  ('b'4),  takes  place  as  described  by  Semon  ('88).     They  arise  by  evagina- 
tions  from  the  outer  side  of  the  circumoral  ring  close  beside  the  outgrowths  which  form 
the  radial  nerves.     With  the  growth  of  the  latter,  the  otocysts  come  to  lie  external  to 
them  at  the  point  where  they  bend  backward,  and  in  this  position  the  sense-organs  remain 
throughout  life.     It  is  a  very  evident  and  noteworthy  fact  that  the  development  of  the 
radial  nerves  and  otocysts  does  not  take  place  at  the  same  time  in  the  five  radii,  but  there 
is  a  marked  difference  between  them.     The  first  to  appear  is  that  nerve  which  subse- 
quently indicates  the  mid-ventral  radius,  and  with  it  appear  its  two  otocysts.     The  two 
lateral  ventral  nerves  appear  next,  and  with  them  the  otocyst  which  accompanies  each  one 
on  its  ventral  side.     The  lateral  dorsal  nerves  next  appear  (Fig.  1),  and  very  soon  after- 
wards the  two  other  otocysts  of  the  right  and  left  ventral  radii  are  formed.     Last  of  all 
to  develop  are  the  otocysts  of    the  right  and  left  dorsal  radii.     This  sequence  in  the 
appearance  of  these  nerves  and  sense-organs  is  probably  connected  with  the  fact  already 
mentioned,  that  the  thickened  ectoderm  which  made  up  the  floor  of  the  atrium  did  not  lie 
symmetrically  around  the  mouth,  but  the  greater  part  of  it  was  posterior  or,  when  the 
mouth  lies  at  the  anterior  end  of  the  larva,  ventral  to  it.     What  the  significance  of  this 
condition  may  be,  I  am  unable  to  suggest,  but  it  is  interesting  to  note  that  in  Cucumaria 
Ludwig  ('91)  found  the  ventral  radius  the  most  advanced  in  development.     Any  possible 
similarity  ends  here  however,  for  the  development  of  the  other  radii  was  quite  the  reverse 
in  Cucumaria  of  what  it  is  in  Synapta. 

About  the  time  of  the  completion  of  the  pentactula  form,  there  appear  in  the  ecto- 
derm of  various  parts  of  the   body  peculiar  invaginations   (Fig.  41)  which  are  finally 


SYNAPTA  VIVIPARA.  67 

connected  with  the  exterior  only  by  a  very  narrow  canal  (Figs.  42  and  43,  and  Fig.  17, 
Igo.) .  These  organs  appear,  from  their- structure  and  the  great  variation  which  they  show 
in  staining,  to  be  of  a  glandular  nature,  and  I  am  inclined  to  think  they  may  be  connected 
with  the  absorption  of  nourishment  from  the  fluid  of  the  body-cavity  of  the  mother,  for 
they  never  increase  in  size,  are  most  abundant  in  the  young  with  ten  tentacles,  and  seem 
to  have  entirely  disappeared  in  the  adult,  and  finally,  nothing  of  the  kind  has  been  described 
for  any  other  holothurian.  When  fully  grown  they  measure  about  fifty  mikrons  in  diam- 
eter and  somewhat  less  in  depth.  They  consist  of  very  long,  clear  cells,  with  nuclei  at 
the  extreme  distal  ends,  surrounding  a  more  or  less  spacious  lumen  which  opens  to  the 
exterior  by  a  narrow  canal  of  ordinary  epithelial  cells.  Sometimes  the  clear  cells  stained 
heavily,  but  often  they  did  not  stain  at  all. 

Up  to  this  time  the  mesenchyme  cells  have  played  no  part  in  the  development  of  the 
larva.  In  Fig.  16,  they  are  shown  as  they  appear  scattered  almost  uniformly  through  the 
segmentation  cavity.  Shortly  after  this,  however,  they  begin  to  gather  around  the  lower 
and  outer  edge  of  the  water-vascular  ring,  and  by  the  time  the  pentactula  stage  is  reached 
they  have  begun  the  formation  of  the  calcareous  ring.  Contrary  to  Semon's  ('88)  views 
on  S.  digitata,  and  in  accordance  with  Ludwig's  ('91)  observations  on  Cucumaria,  I  have 
found  no  evidence  at  all  of  any  mesenchymatous  musculature  on  the  oesophagus.  The 
first  products  of  the  mesenchyme  cells  to  appear  are  five  small  straight  rods  between  the 
primary  tentacles  but  outside  and  somewhat  below  the  hydrocoel  ring.  Soon  after  these, 
five  more  appear  below  the  bases  of  the  tentacles,  so  that  there  are  now  ten  rods,  five 
radial  and  five  interradial,  and  they  continue  in  this  position  so  long  as  there  are  only  five 
tentacles.  I  saw  no  evidence  at  all  of  any  such  shifting  of  position  of  the  first  five  rods 
as  Semon  ('88)  records  for  S.  digitata,  nor  could  I  consider  the  position  of  the  second 
series  as  agreeing  at  all  with  his  description.  Very  soon  after  the  appearance  of  the  ten 
rods,  they  fork  at  the  ends  and  begin  to  branch  very  irregularly.  As  will  be  seen  from 
Fig.  46  a-i,  the  divisions  occur  at  all  sorts  of  angles  and  not  only  differ  decidedly  from 
Semon's  figures  of  the  same  rods  in  S.  digitata  but  show  no  sign  at  all  of  following  his 
law  ('87)  for  the  formation  of  calcareous  plates  in  Echinoderms.  The  much-branched 
ends  of  the  rods  come  into  very  close  contact  but  evidently  never  mingle,  for  the  plates 
into  which  they  develop  are  at  all  times  easily  separable  and  the  line  of  division  between 
them  is  practically  straight.  Very  soon  after  the  appearance  of  the  first  five  calcareous 
rods,  the  radial  nerves  grow  backward  over  them  and  before  the  completion  of  the  pentac- 
tula, run  to  the  posterior  end  of  the  animal. 

We  have  now  reached  the  complete  pentactula  form,  a  slightly  older  stage  of  which 
appears  in  Fig.  18.  The  pentactula  is  about  half  a  millimeter  long  and  its  characteristics 


68  HUBERT  LYMAN    CLARK   ON 

nfay  be  briefly  summed  up  as  follows:  —  Water- vascular  system  consisting  of  a  closed 
hydrocoel  ring  or  circumoral  water-tube  with  five  primary  tentacles,  between  which  are 
five  very  much  smaller  but  equally  erect  secondary  outgrowths;  a  water-tube  in  the  mid- 
dorsal  interradius  connecting  the  circumoral  ring  with  the  exterior;  and  a  Polian  vessel 
in  the  left  dorsal  interradius.  Nervous  system  consisting  of  a  circumoral  ring ;  five  ten- 
tacle nerves  on  the  inner  face  of  the  primary  tentacles,  the  ectoderm  of  which  is  consid- 
erably thickened,  especially  on  the  outer  side ;  five  radial  nerves  bending  backward  over 
the  secondary  outgrowths  of  the  water-ring  and  over  the  radial  pieces  of  the  calcareous 
ring,  and  running  to  the  posterior  end  of  the  body;  and  five  pairs  of  otocysts,  lying 
external  to  the  radial  nerves,  where  they  bend  backward.  Digestive  system,  consisting 
of  a  short  oesophagus  with  the  mouth  opening  anteriorly  in  the  center  of  the  circle  of 
tentacles,  a  large  stomach,  a  comparatively  short  intestine  with  a  single  loop  in  it,  and 
usually  an  anus  formed  secondarily  near  or  at  the  aboral  pole.  Digestive  system  attached 
to  the  wall  throughout  its  whole  course  by  a  mesentery,  formed  by  the  union  of  the  two 
walls  of  the  right  and  left  coelomic  pouches.  Calcareous  ring  consisting  of  five  radial 
and  five  interradial  pieces  with  much-branched  ends.  A  few  scattered  glandular  organs 
of  doubtful  function  in  various  parts  of  the  ectoderm. 

7.   THE  DEVELOPMENT  OF  THE  ADULT  SYNAPTA. 

Since  there  is  no  cessation  of  growth  nor  any  resting  period  on  the  assumption  of 
the  pentactula  form,  it  is  impossible  to  draw  any  hard  and  fast  lines  which  will  always 
serve  to  distinguish  that  stage.  For  many  larvae,  which  appear  to  have  only  five  tenta- 
cles, show  on  careful  examination  the  rudiments  of  new  ones,  and  other  larvae  which 
show  no  tentacles  externally  show  the  perfect  pentactula  form,  when  sectioned.  As  soon 
as  the  five  primary  tentacles  have  pushed  out  so  far  as  to  entirely  obliterate  the  original 
opening  of  the  atrium,  the  secondary  outgrowths  of  the  water-ring,  which  have  hitherto 
scarcely  shown  any  indication  of  growth,  begin  to  develop  and  push  upward.  As  we  have 
already  seen,  the  radial  nerves  lie  directly  over  them,  so  that  they  cannot  grow  straight 
up  but  push  out  to  one  side  or  the  other  of  the  nerve.  The  outgrowth  which  lies  in  the 
mid-ventral  radius,  however,  develops  very  slightly  and  does  not  normally  push  out  on 
either  side  of  the  nerve  which  overlies  it.  The  outgrowths  which  lie  in  the  right  and 
left  ventral  radii  take  the  opposite  course,  and  broadening  out  laterally,  grow  up  on  both 
sides  of  the  nerves  which  overlie  them,  to  form  accessory  tentacles.  The  outgrowth  of 
the  right  dorsal  radius  pushes  out  on  the  dorsal  side  of  its  overlying  nerve  and  forms  an 
accessory  tentacle  in  the  mid-dorsal  interradius,  while  the  outgrowth  of  the  left  dorsal 


SYNAPTA   VIVIPARA.  69 

radius  shows  as  yet  little  tendency  to  develop  either  way  (Fig.  85) .  Consequently  we 
now  have  a  larval  form  with  ten  tentacles,  two  in  each  interradius.  As  the  accessory 
tentacles  grow  very  rapidly  they  are  soon  equal  in  size  to,  and  cannot  be  distinguished 
from,  the  five  primary  tentacles.  It  is  hard  to  decide  positively  which  of  the  five  accessory 
tentacles  develops  first,  for  apparently  they  all  begin  to  grow  at  about  the  same  time. 
In  Chirodota  rotifera,  Ludwig  ('81)  found  the  first  two  accessory  tentacles  in  the  lateral 
dorsal  interradii,  and  he  does  not  speak  of  finding  any  trace  of  additional  tentacles  in  the 
other  interradii.  I  have  not  found  any  stage  similar  to  that  in  S.  vivipara,  and  I  think 
the  five  accessory  tentacles  appear  at  practically  the  same  time.  It  is  an  important  and 
interesting  fact,  however,  that  the  five  accessory  tentacles  are  formed  in  precisely  the 
same  manner  and  from  the  same  radii  as  the  second  series  of  five  tentacles  in  Cucumaria, 
(Ludwig,  '91).  It  seems  to  me  that  this  fact  proves  satisfactorily  that  the  radial  canals 
in  Synaptidae  are  homologous  with  those  of  the  other  holothurians  or,  more  accurately, 
the  secondary  outgrowths  of  the  hydrocoel  ring  in  the  Synaptidae  are  homologous  with 
the  five  outgrowths  of  the  hydrocoel  ring  in  the  true  holothurians.  In  both  cases,  the 
ten-tentacled  young  has  one  primary  and  one  accessory  tentacle  in  each  interradius. 
While  this  change  is  taking  place  in  the  number  and  arrangement  of  the  tentacles,  a  cor- 
responding change  is  going  on  in  the  calcareous  ring.  As  the  accessory  tentacles  push 
out  into  the  interradii,  the  calcareous  rod  which  lies  at  the  base  of  the  primary  tentacle 
cornes  to  lie  between  it  and  the  accessory  tentacle.  I  could  not  see  that  this  came  about 
by  any  actual  movement  of  the  rod  itself,  but  was  due  simply  to  the  increase  of  width 
in  each  interradius.  In  the  further  growth  of  the  calcareous  ring,  the  interradial  pieces 
send  up  projections  between  the  two  tentacles  (Fig.  46  h)  and  at  the  same  time  branch 
and  divide  so  rapidly  and  irregularly  that  they  soon  become  plates,  with  straight  sides 
but  pointed  anteriorly  and  notched  behind,  made  up  of  a  very  fine  irregular  network  of 
calcareous  strands  (Fig.  44).  The  radial  plates  develop  in  the  same  way  but  send  up  two 
projections,  one  on  each  side  of  the  radial  nerve  (Fig.  46  i).  which  finally  fuse  together 
above  it  and  thus  form  the  perforated  plates  of  the  ring  (Fig.  45).  About  the  same 
time,  the  mesenchyme  cells  lying  between  the  ectoderm  and  the  wall  of  the  coelom 
begin  to  gather  in  groups  close  to  the  ectoderm  and  there  give  rise  to  anchors 
and  anchor-plates  so  characteristic  of  the  adult  Synapta.  The  development  of  the  cal- 
careous bodies  from  a  straight  rod  takes  place  as  described  by  Semon  ('87)  for  Synapta 
inhaerans.  While  these  deposits  appear  in  the  body-wall  as  far  anteriorly  as  the  base  of 
the  tentacles,  in  the  walls  of  the  latter,  lying  parallel  to  the  long  axis,  there  appear 
numerous  rather  long,  more  or  less,  knobbed  rods  (Fig.  48)  similiar  to  those  described 


70  HUBERT  LYMAN  CLARK  ON 

by  Seruon  ('87)  from  the  tentacles  of  various  synaptids.  These  become  very  abundant 
in  the  older  ten-tentacled  larvae. 

The  digestive  tract  meantime  has  increased  in  length,  and  the  stomach  is  more 
clearly  marked  off  from  the  intestine  and  oesophagus.  The  nervous  system  has  not 
undergone  any  marked  changes,  but  each  of  the  accessory  tentacles  is  supplied  with  a 
nerve  on  its  inner  side,  as  in  the  case  of  the  primary  tentaclesi  In  various  parts  of  the 
skin,  especially  anteriorly,  clusters  of  ectoderm  cells  are  to  be  found  which  later  form 
the  so-called  sense-papillae  ("  Tastpapillen  "  of  Hamann, '83).  I  have  been  unable  to 
find  any  connection  between  these  spots  and  the  nerves  until  a  very  much  later  period, 
and  I  cannot  decide  how  or  when  this  connection  is  made.  The  glandular  organs 
previously  described  are  very  abundant  at  this  stage,  especially  posteriorly.  Soon  after 
the  pentactula  form  is  complete,  the  walls  of  the  coelom  and  of  the  hydrocoel  begin  the 
formation  of  muscle  fibers,  always  on  the  side  turned  from  the  cavity  which  they  enclose. 
The  first  to  appear  are  the  longitudinal  muscles  of  the  tentacles  and  radii.  The  former 
appear  as  fibers  on  the  outside  of  the  tentacle  canals  and  they  soon  form  quite  a  thick 
layer.  The  radial  muscles  arise  within  a  fold  of  the  coelomic  wall,  which  appears  along 
the  inner  side  of  the  radial  nerves.  This  fold  begins  anteriorly  near  the  calcareous  ring 
and  runs  backwards  with  the  nerve,  enclosing  a  considerable  space  between  its  walls.  In 
this  space  the  muscle  fibers  arise  from  the  endodermal  cells  of  the  coelom.  Later  on, 
the  circular  muscles  of  the  body  appear,  arising  from  the  outer  side  of  the  coelomic  wall 
also.  They  cross  the  space  in  which  the  longitudinal  muscles  lie,  forming  a  layer 
between  the  latter  and  the  nerve.  At  the  same  time,  the  longitudinal  and  circular 
muscles  of  the  digestive  tract,  and  the  muscles  of  the  water-ring,  begin  to  appear,  so  that 
by  the  time  the  ten-tentacled  stage  is  reached,  the  musculature  is  practically  that  of  the 
adult.  With  the  appearance  of  the  longitudinal  muscles  of  the  tentacles,  comes  the 
development  of  the  valves  at  the  openings  of  the  tentacular  canals,  close  to  the  upper 
edge  of  the  calcareous  ring. 

Before  the  accessory  tentacles  have  begun  to  appear,  there  arises  on  the  right  hand 
side  of  the  mesentery  which  fastens  the  intestine  to  the  wall  of  the  left  interradius  a 
longitudinal  fold  or  evagination  of  the  epithelium  close  to  the  intestine  (Fig.  58).  This 
fold  follows  the  course  of  the  intestine,  with  the  mesentery  on  the  dorsal  side,  and  grows 
forward  along  the  stomach  and  backward  toward  the  anus.  Later  a  similar  fold  appears 
in  the  coelomic  epithelium  on  the  opposite  (ventral)  side  of  the  digestive  tract  and  the 
two  folds  soon  become  connected  around  the  intestine  and  stomach  by  numerous 
ill-defined  lacunae  between  the  coelomic  wall  and  that  of  the  digestive  organ  itself. 
These  vessels  are  the  first  stages  of  the  blood  vascular  system  and  into  them  cells  from 


SYNAPTA    VIVIPARA.  71 

the  coelomic  epithelium  pass  to  form  the  blood  corpuscles.  Theoretically  the  vessels 
ought  to  be  lined  with  connective  tissue  of  mesenchyme  cells  but  these  are  so  few  in  the 
early  stages  of  the  larva,  that  the  connective  tissue  between  the  laminae  of  the  mesen- 
teries or  even  around  the  digestive  tract  is  very  hard  to  demonstrate.  The  main  blood 
vessels  appear  to  be  purely  entodermal  in  origin  and  their  walls  seem  to  be  made  up 
solely  of  the  coelomic  wall.  At  any  rate,  the  dorsal  vessel  does  not  arise  in  S.  vivipara 
iu  the  way  given  by  Semon  ('88)  for  its  origin  in  S.  digitata,  by  a  simple  split  in  the 
mesenchyme  where  the  two  coelomic  folds  unite  above  the  intestine  to  form  the 
mesentery. 

Very  soon  after  the  pentactula  stage  is  reached  the  first  rudiment  of  the  genital 
system  appears.  It  arises  on  the  inner  side  of  the  right-hand  lamina  of  the  dorsal 
mesentery  between  the  stone-canal  and  the  oesophagus.  The  first  appearance  is  simply 
the  increased  size  of  the  cells  at  this  point,  resulting  in  a  thickening  of  the  wall  (Fig. 
32),  but  the  cells  soon  multiply  rapidly  and  form  a  more  or  less  spherical  mass  within 
the  mesentery  (Fig.  33).  As  this  mass  increases  in  size,  cavities  appear  within  it 
(Fig.  34)  and  these  increase  in  size  and  begin  to  unite  together  until  they  form  one 
central  lumen  for  the  gland  (Fig.  35).  Meanwhile  the  right  lamina  of  the  mesentery 
forms  an  outer  epithelium  which  soon  becomes  quite  distinct  from  the  central  mass  of 
cells.  Between  this  epithelium  and  the  remainder  of  the  gland  a  few  mesenchyme  cells 
form  an  extremely  scanty  connective-tissue  layer.  In  the  full-grown  ten-tentaculed 
larva,  the  genital  gland  is  plainly  seen  (Fig.  19),  lying  entirely  on  the  right-hand  side 
of  the  mesentery,  near  the  stone-canal. 

The  ten-tentacled  larva  (Fig.  19)  is  a  more  clearly  defined  stage  in  the  development 
of  Synapta  vivipara  than  is  the  pentacula;  that  is,  it  seems  to  last  longer,  and  the  rela- 
tive condition  of  development  of  the  various  organs  is  more  constant.  The  changes 
which  occur  subsequently  in  the  assumption  of  the  adult  form  must  now  be  considered. 
The  most  obvious  of  these  is  the  increase  in  the  number  of  tentacles  which  arises  from 
the  addition  of  another  tentacle  to  the  right  and  left  dorsal  interradii  (Fig.  86).  The 
extra  tentacle  of  the  left  side  arises  from  the  left  dorsal  "  secondary  outgrowth  "  of  the 
hydrocoel  ring,  which  has  hitherto  remained  in  its  original  position  beneath  the  left 
dorsal  nerve  but  now  pushes  out  on  its  lower  or  ventral  side  and  forms  the  eleventh  ten- 
tacle. At  the  same  time,  the  extra  tentacle  of  the  right  side  arises  from  the  lower  or 
ventral  side  of  the  right  dorsal  "  secondary  outgrowth  "  which  pushes  out  on  that  side 
of  the  right  dorsal  nerve  and  forms  the  twelfth  tentacle.  Not  infrequently  individuals 
are  found  with  thirteen  tentacles,  and  in  these  the  extra  tentacle  is  usually  in  one  of  the 
ventral  interradii.  In  such  cases  it  is  probable  that  the  mid-ventral  "  secondary  out- 


72  HUBERT  LYMAN  CLARK  ON 

growth"  has  grown  up  on  one  side  or  the  other  of  the  mid-ventral  nerve,  although  in 
normal  specimens  it  does  not  develop  at  all.  Occasionally  the  extra  tentacle  is  in  the  mid- 
dorsal  interradius,  and  in  such  cases  it  is  probable  that  the  left  dorsal  "  secondary 
outgrowth,"  which  normally  develops  only  a  single  tentacle,  has  given  rise  to  a  second 
on  the  dorsal  side  of  the  left  dorsal  nerve.  Corresponding  to  these  changes  in  the  num- 
ber of  tentacles  additional  plates  appear  in  the  calcareous  ring,  but  these  plates  do  not 
arise  by  interpolation  of  new  rods.  On  the  contrary,  the  plate  of  the  same  radius  with 
the  new  tentacle  increases  its  length  and  sends  upward  a  new  projection  for  the  support 
of  the  tentacle  (Fig.  47),  and  this  subsequently  forms  the  center  of  the  new  accessory 
plate.  In  specimens  of  the  calcareous  rings  cleaned  with  caustic  soda,  it  seemed  to  me 
that  the  calcareous  plates  of  the  right  and  left  dorsal  radii  were  less  easily  separated  from 
those  plates  on  their  ventral  side  than  from  those  on  the  dorsal,  so  I  am  inclined  to  think 
that  for  a  time,  if  not  throughout  life,  these  two  plates  remain  in  closer  union  than  any 
of  the  others.  In  specimens  with  thirteen  tentacles,  there  is  an  additional  plate  in  the 
calcareous  ring  corresponding  to  the  extra  tentacle.  About  this  same  time  the  miliary 
granules  (Fig.  50)  begin  to  appear  in  various  parts  of  the  body-wall  and  in  the  tentacles. 
Like  all  other  calcareous  concretions  of  Synapta,  they  are  formed  by  mesenchyme  cells. 
They  usually  appear  in  clusters  of  several  hundred,  which  continue  to  increase  in  number 
afterwards  until  it  may  reach  thousands.  By  the  time  twelve  tentacles  have  appeared, 
the  genital  gland  begins  to  push  over  on  the  left  hand  side,  but  it  is  not  until  long  after 
the  adult  form  is  assumed  in  all  other  respects  that  the  left  branch  of  the  gland  equals  the 
right  in  size.  As  soon  as  the  left  branch  is  well  started,  the  germinal  epithelium  begins 
to  push  upward  in  the  mesentery  beside  the  stone-canal,  and  forms  the  genital  duct,  but 
does  not  reach  the  outer  body-wall  for  some  time.  This  account  of  the  development  of 
the  genital  duct  accords  with  Mortensen's  ('94)  observations  on  Cucumaria  glacialis, 
although  his  account  of  the  origin  of  the  genital  gland  itself  differs  considerably  from  my 
observations.  Important  changes  are  going  on  meanwhile  in  the  nervous  system.  From 
the  inner  side  of  the  circumoral  ring  nerves  or  bands  of  nervous  tissue  arise  and  pass 
inward  to  the  oesophagus.  These  will  be  referred  to  more  fully  in  describing  the  nervous 
system  of  the  adult.  Even  in  the  ten-tentacled  stage,  before  the  remaining  two  tentacles 
have  made  a  fair  start,  there  arises  on  each  side  of  the  tentacle  nerve  at  its  base  a  knob- 
like  outgrowth  which  becomes  covered  over  with  a  peculiar  mesenchyme  layer,  and  these 
form  the  "  eyes,"  which  also  appear  at  the  base  of  the  eleventh  and  twelfth  tentacles, 
after  they  receive  their  nerves  from  the  circumoral  ring.  With  the  appearance  of  these 
eyes,  the  first  trace  of  pigment  appears  in  the  mesenchyme  not  only  about  them  but  in 
various  parts  of  the  body,  especially  around  the  calcareous  ring.  This  pigment  on  its 


SYNAPTA  VIVIPARA.  73 

first  appearance  is  bright  green,  even  about  the  eyes,  so  that  at  this  stage  the  eyes  of 
the  young  Synapta  are  very  conspicuous  as  large  green  spots  at  the  base  of  the  tentacles. 
Very  soon,  however,  a  dark  reddish  brown  pigment  appears,  but  this  is  probably  an  older 
stage  of  the  green,  and  not  a  different  pigment;  for  the  pigment  around  the  eyes  soon 
loses  its  green  color  and  turns  brown,  and  there  is  no  reason  to  assume  that  the  pigment 
in  other  parts  of  the  body  is  any  different  from  that  around  the  eyes.  All  of  the  pigment 
arises  in  the  connective  tissue,  and  is  apparently  a  product  of  the  mesenchyrne  cells.  It 
is  especially  abundant  at  the  anterior  end  of  the  body,  and  above  all  other  places  in  and 
around  the  calcareous  ring. 

Before  the  number  of  tentacles  is  complete  the  ciliated  funnels  so  characteristic  of 
Synapta  begin  to  appear  on  the  mesentery,  near  the  body-wall.  These  funnels  arise 
from  a  large  cell  or  group  of  cells  in  the  endodennal  epithelium  of  the  mesentery  (Figs. 
61  and  62a).  The  multiplication  of  these  cells  soon  forms  a  hemispherical  outgrowth 
(Fig.  G2d)  which  increases  in  size  and  becomes  more  and  more  spherical  in  shape,  until 
it  is  finally  attached  to  the  mesentery  by  only  a  narrow  stalk  (Fig.  62e).  It  then  begins 
to  flatten  on  one  side,  and  the  cells  of  its  outer  layers  become  smaller  and  stain  more 
heavily  than  those  nearer  the  stalk  (Fig.  62f).  The  flattened  surface  at  last  becomes 
concave  and  the  funnel  shape  begins  to  be  assumed.  At  the  same  time,  the  stalk 
becomes  elongated  and  draws  up  within  it  some  of  the  connective-tissue  layer  of  the 
mesentery,  which  becomes  the  supporting  layer  of  the  funnel.  Even  during  the  ten- 
tentacled  stage  the  digits  of  the  tentacles  begin  to  be  formed,  but  they  do  not  become 
prominent  until  the  twelve  tentacles  are  all  developed.  The  digits  arise  as  evaginations 
of  the  water-canal  of  the  tentacle  (Fig.  56)  which  very  soon  become  shut  off  from  the 
main  canal  and  in  the  adult  have  no  connection  with  it  (Fig.  55).  The  earliest  ones  to 
appear  are  near  the  middle  of  the  tentacle,  and  the  later  ones  appear  both  proximally 
and  distally  to  them.  The  digits  form  longitudinal  muscles  on  the  outer  side  of  the 
central  cavity  in  the  same  way  as  the  tentacles  themselves.  They  are  also  supplied  with 
nerves  from  the  main  tentacle  nerve.  The  peculiar  glandular  organs  of  the  larva  are  no 
longer  forming  but  seem  rather  to  be  disappearing,  and  the  longitudinal  rods  of  the 
tentacles  reach  their  maximum  number  at  this  time.  The  circumoral  sinus,  which  was 
entirely  cut  off  from  the  rest  of  the  coelom  in  the  pentactula  stage,  has  increased  greatly 
in  size  (Fig.  89)  but  is  now  in  open  communication  with  the  body-cavity,  though  strands 
of  connective  tissue  traverse  it,  uniting  the  oesophagus  to  the  water-ring  and,  higher  up, 
to  the  coelomic  wall.  With  the  greatly  increased  size  of  the  young  Synapta,  comes  a 
considerable  change  in  the  relative  position  of  the  organs  in  the  body-cavity.  The  body 
has  grown  much  posteriorly,  drawing  out  with  it  that  part  of  the  intestine  which  lies  in 


74  HUBERT    LYMAN    CLARK   ON 

the  right  ventral  interradius.  The  nerve  ring  is  drawn  upward  with  the  growth  of  the 
tentacles,  so  that  it  comes  to  lie  very  near  the  ectoderm  at  their  base.  The  increased 
length  of  the  tentacular  canals  has  pushed  the  water-ring  downward  so  that  it  lies  some 
distance  below  the  calcareous  ring  (Figs.  90  .and  91),  but  it  is  still  in  open  communica- 
tion with  the  exterior  by  means  of  the  water-canal  (Fig.  66).  Mesenchyme  cells  have 
formed  a  few  calcareous  rods  about  the  latter  (Fig.  49),  especially  near  the  point  where 
it  passes  into  the  body-wall.  Just  within  the  body-cavity  from  this  point,  openings  have 
appeared  on  it  which  place  its  interior  in  direct  communication  with  the  body-cavity,  so 
that  the  water-vascular  system  combines  the  primitive  external  opening  of  the  pentactula 
with  the  internal  madreporitic  openings  of  the  other  holothurians.  The  mesenchyme 
cells  around  the  calcareous  ring  have  formed  on  its  posterior  edge  a  connective-tissue 
ring,  which  later  becomes  so  prominent  in  the  adult  as  the  cartilaginous  ring. 

In  concluding  this  account  of  the  embryology  it  may  be  well  to  summarize  briefly 
the  derivation  of  the  different  organs  from  the  germ  layers  of  the  gastrula. 

Ectoderm.  From  the  gastrula-ectoderm  arise  the  ectoderm  of  the  adult,  the  sensory 
epithelium  of  the  tentacles,  the  entire  nervous  system  including  all  the  sense  organs, 
the  larval  glandular  organs,  and  a  small  part  of  the  oesophagus.  Possibly  the 
extreme  posterior  part  of  the  rectum  is  also  ectodermal. 

Endoderm.  From  the  gastrula-endoderm  arise  first  of  all  the  scattered  mesenchyme  cells 
which  make  up  the  mesoderm.  Soon  afterwards  the  hydroenterocoel  is  divided  off. 
The  remainder  of  the  archenteron  forms  simply  the  lining  of  the  digestive  tract, 
including  most  of  the  oesophagus.  From  the  hydroenterocoel,  the  coelomic  pouches 
are  constricted  off,  leaving  behind  the  hydrocoel,  from  which  the  entire  water- 
vascular  system,  and  also  the  cavities  of  the  digits,  arise.  The  longitudinal  muscles 
of  the  tentacles  and  digits  come  from  the  epithelium  of  the  hydrocoel.  The  coelomic 
pouches  form  the  peritoneal  lining  of  the  body-cavity  and  the  epithelial  covering  for 
the  various  organs  contained  in  it.  All  the  muscles  of  the  body-wall,  gut,  genital 
glands,  water-ring,  and  Polian  vessels  are  also  derivatives  of  the  endoderm.  The 
genital  organ,  including  the  genital  duct,  and  the  ciliated  funnels  are  likewise 
derived  from  the  wall  of  the  coelom.  The  haemal  system  is  also  covered  by  the 
epithelium  of  the  coelom  and  apparently  arises  as  evaginations  of  the  same,  while 
the  blood-corpuscles  certainly  come  from  that  layer. 

Mesoderm.  From  the  mesenchyme  cells,  arising  from  the  archenteron  of  the  gastrula, 
come  all  the  connective  tissue  of  the  body,  the  pigment,  the  covering  of  the  eyes, 
all  the  calcareous  concretions  (including  the  calcareous  ring),  and  the  cartilaginous 


SYNAPTA    VIVIPARA.  75 

ring.  No  trace  of  mesenchymatous  musculature  was  found  anywhere,  and  the  part 
which  the  mesoderm  takes  in  the  formation  of  the  haemal  system  is  certainly  incon- 
siderable. 


8.  THE  ANATOMY  OF  THE  ADULT. 

Although  the  anatomy  of  the  European  Synaptas  is  so  well  known,  thanks  to  the 
investigations  of  Baur  ('64),  Semon  ('87),  Hamann  ('83  and  '89),  Cue"not  ('91),  and 
others,  there  are  so  many  points  in  which.  Synapta  vivipara  differs  from  the  forms 
hitherto  examined,  it  seems  desirable  to  add  a  few  words  concerning  these  and  other 
points.  Except  in  the  case  of  sense-organs,  no  attempt  has  been  made  to  go  into  the 
histology,  but  my  attention  has  been  confined  to  the  more  general  features  of  the  minute 
anatomy.  In  the  structure  of  the  body-wall  and  the  muscular  system,  there  are  no 
important  features  to  mention,  aside  from  the  shape  of  the  longitudinal  radial  muscle 
bands.  Each  of  these  bands  is  forked  at  its  anterior  extremity,  and  the  two  branches 
are  attached  to  the  radial  calcareous  plate,  one  on  each  side  of  the  radial  nerve.  These 
branches  soon  unite  as  they  pass  backward,  and  form  a  single  narrow  band,  which 
extends  far  out  into  the  body-cavity.  But  still  further  back,  it  decreases  in  depth  and 
increases  correspondingly  in  width,  and  the  epithelium  which  covers  it  tends  to  fuse  at 
the  outer  edges  with  the  epithelium  of  the  body-cavity,  so  that  at  numerous  points  in  its 
course  the  muscle  has  acquired  secondary  attachments  to  the  body-wall.  During  the 
greater  part  of  its  course,  it  is  a  nearly  flat  band,  but  as  it  approaches  the  extreme 
posterior  region  of  the  body,  it  tends  to  become  cylindrical,  and  where  it  ends  near  the 
anus  the  cross-section  is  circular.  These  changes  in  shape  will  be  made  clear  from  Figs. 
94-100.  The  structure  of  the  genital  glands  has  already  been  given  in  detail,  and  the 
openings  in  the  wall  of  the  rectum  have  also  been  sufficiently  described.  The  blood- 
vascular  or  haemal  system  is  very  simple,  consisting  of  a  dorsal  and  ventral  vessel  on  the 
intestine  and  stomach  with  connecting  lacunae  in  their  walls.  Posteriorly,  both  vessels 
end  about  half  way  down  that  section  of  the  intestine  which  lies  in  the  right  ventral 
interradius  (Fig.  92) .  Anteriorly,  the  ventral  vessel  ends  a  little  in  front  of  the  stomach, 
on  the  oesophagus.  The  dorsal  vessel  runs  forward  to  the  water-ring  and  forms  on  its 
inner  side  a  circumoesophageal  ring,  from  which  branches  pass  on  to  each  tentacular 
vessel.  The  dorsal  blood-vessel  also  seems  to  open  out  in  the  mesentery  to  form  broad 
lacunae  about  the  genital  gland,  such  as  Cuenot  ('91)  found  in  European  Synaptas,  but  I 
never  found  coagulated  blood  there  as  in  the  dorsal  vessel,  and  I  do  not  feel  sure  that 


76  HUBERT  LYMAN  CLARK  ON 

such  lacunae  actually  exist.  The  ventral  vessel  of  the  stomach  does  not  lie  appressed  to 
its  wall,  but  entirely  free  from  it  and  connected  with  it  by  several  small  branches.  It  is 
also  connected  by  a  large  transverse  vessel  with  the  ventral  vessel  of  the  intestine  (Fig. 
92),  and  the  two  sections  of  the  latter  are  also  connected  by  a  similar  vessel.  These 
transverse  vessels  do  not  appear  until  the  animal  is  several  centimeters  long,  when  they 
arise  by  outgrowths  of  the  coelomic  epithelium  of  stomach  and  intestine  which,  lying 
close  together  as  they  do  in  the  loops  of  the  digestive  tract,  touch  and  fuse  (Figs.  59  and 
60)  and  with  the  increased  growth  of  the  intestine  are  finally  drawn  out  to  slender  con- 
necting vessels.  Like  the  vessels  of  the  young  Synapta,  these  are  supposed  to  be  lined 
with  connective  tissue,  but  I  have  been  unable  to  detect  it  in  their  walls. 

The  ciliated  funnels  of  Synapta  vivipara  differ  considerably  in  appearance  from  those 
of  S.  digitata  or  S.  inhaerans,  though  they  do  not  differ  essentially  in  structure.  Only 
one  sort  seems  to  be  present  and  these  are  quite  small  but  extremely  numerous  on  all 
three  of  the  mesenteries.  They  measure  from  40/x  to  75/t  in  length,  and  from  30/x  to 
4(V  in  breadth  and  depth,  which  is  only  about  half  the  size  of  those  of  /S.  diyitata.  They 
are  broad  funnel-  or  cornucopia-shaped  in  outline  and  usually  have  a  short  stalk. 
Their  general  structure  will  be  easily  understood  from  Figs.  63-65.  The  water-vascular 
system  consists  of  a  circumoesophageal  ring  from  which  canals  arise  and  pass  to  the  ten- 
tacles, into  which  their  entrance  is  guarded  by  well-developed  valves.  Each  tentacle 
rests  on  the  calcareous  ring  in  such  a  way  that  the  outer  half  of  the  base  is  on  the  out- 
side of  the  calcareous  plate,  forming  a  sort  of  rudimentary  ampulla  (Fig.  90).  There  is 
not  in  the  adult,  any  more  than  in  any  of  the  larval  stages,  the  slightest  trace  of  radia. 
water-canals.  Dependent  from  the  ring-canal  there  is  always  present  in  the  left  dorsal 
interradius  a  slender  Polian  vessel  five  or  six  millimeters  long,  and  in  nearly  all  adults 
additional  Polian  vessels,  sometimes  as  many  as  six,  are  present  in  the  ventral  interradii. 
The  stone-canal  leaves  the  water-ring  on  the  left-hand  side  of  the  mid-dorsal  interradius 
and  does  not  lie  in  the  dorsal  mesentery  but  clearly  separate  from,  and  to  the  left  of  it. 
It  soon  passes  into  it,  however,  on  its  outward  course  and  runs  to  the  body-wall  close 
beside  the  genital  duct  (Figs.  66  and  67).  It  enters  the  body-wall  on  the  right  of  the 
mesentery  and  bends  upward  more  or  less  abruptly,  opening  finally  to  the  exterior  close 
behind  the  circle  of  tentacles  (Figs.  66-70).  In  exceptional  cases  there  are  two  open- 
ings (Figs.  71-73)  or  rarely  the  reverse  happens  and  the  canal  closes  before  the  exterior 
is  reached.  Besides  this  external  opening,  the  stone-canal  also  opens  into  the  body-cavity 
through  a  well-developed  madrepore  (Figs.  66  and  74).  Throughout  its  course  the  canal 
is  heavily  ciliated,  and  especially  so  around  these  madreporitic  openings,  the  whole 
arrangement  being  admirably  adapted  for  keeping  the  body-cavity  fluid  well  aerated. 


SYNAPTA   VIVIPARA.  77 

The  nervous  system  consists  as  in  all  Synaptidae  of  the  central  circumoral  ring  with  the 
five  radial  branches  and  the  smaller  branches  to  each  of  the  tentacles,  but  there  are  some 
additional  nerves  and  certain  of  the  sense-organs  which  have  not  been  figured  hitherto. 
Each  of  the  radial  nerves  is  divided  longitudinally  into  an  outer  and  an  inner  band  as  in 
other  Synaptas,  but,  unlike  them,  there  are  no  canals  or  vessels  of  any  kind  accompanying 
the  nerves.  There  are,  therefore,  in  the  radii  of  S.  vivipara,  no  spaces  or  lacunae  in 
connection  with  either  the  blood,  water,  or  nervous  systems,  but  they  are  marked  simply 
by  the  longitudinal  muscles  and  nerves  (Fig.  99).  Each  tentacle-nerve  sends  off  branches 
to  the  digits  (Fig.  55),  so  that  almost  the  whole  surface  of  the  tentacle  becomes  sensory. 
On  the  base  of  the  tentacles  and  in  various  parts  of  the  ectoderm  all  over  the  body,  there 
are  numerous  sense-buds  or  "  taste-papillae,"  (Fig.  84),  such  as  were  first  described  by 
Hamann  ('83).  The  structure  of  these  organs  has  been  well  described  by  him  and  still 
more  recently  by  Cuenot  ('91).  My  observations  support  the  opinion  of  the  latter,  that 
under  each  one  of  these  sense  papillae  there  lies  a  small  ganglion.  From  the  lower  side 
of  the  circumoral  ring,  there  arises  between  every  two  tentacles  a  broad  band-like  nerve 
(Figs.  75  and  76)  which  runs  inwards  towards  the  mouth,  innervating  the  ectoderm  of  the 
oral  disc  as  well  as  the  muscles  of  the  oesophagus.  Hamann  ('83)  describes  a  single 
nerve  to  the  oesophagus,  and  Semon  ('88)  speaks  of  it  in  S.  digitata,  but  so  far  as  I  can 
learn  no  other  nerves  from  the  inner  side  of  the  ring  have  been  described  in  holothurians. 
At  the  base  of  each  tentacle,  there  are  easily  seen  a  pair  of  reddish  brown  spots,  the  so- 
called  eyes  (Figs.  77  and  78).  Similar  spots  are  mentioned  in  various  Synaptas  by 
Miiller  ('50),  Baur  ('64),  and  Semper  ('68),  but  Semon  ('87)  and  Hamann  ('84)  seem 
to  doubt  their  visual  function.  There  can  be  little  doubt,  however,  that  in  Synapta 
vivipara  these  eyes  are  actually  of  service  as  light-detecting  organs.  In  position  and 
general  structure  they  resemble  those  described  by  Ludwig  and  Barthels  ('91)  for  Synapta 
oittata.  They  consist  of  a  distinct,  rather  horny  mesodermal  layer,  of  a  light  brown 
color,  containing  scattered  nuclei,  overlying  the  swollen  end  of  a  large  nerve  which  arises 
on  each  side  from  the  base  of  the  tentacle-nerve  (Fig.  77).  The  ends  of  these  nerves 
are  made  up  of"  large  nerve-cells  with  large  nuclei,  which  are  somewhat  swollen  and 
apparently  vacuolated  at  their  outer  extremities.  They  are  polygonal  in  outline,  when 
seen  in  cross-section  (Fig.  79),  and  the  inner  ends  taper  off  into  fibers  which  run  out  into 
the  nerve  (Fig.  80) .  The  mesodermal  covering,  which  also  has  the  appearance  of  being 
vacuolated,  is  clearly  a  continuation  of  the  thin  mesoderm  layer  which  surrounds  all  the 
nerves.  The  eyes  are  about  60^  in  diameter,  the  mesodermal  covering  being  six  or  eight 
mikrons  thick.  That  this  covering  may  be  affected  by  light  is  probable,  for  its  color  is 
due  to  the  pigment  it  contains.  The  other  noteworthy  sense-organs  are  the  otocysts 


78  HUBERT  LYMAN    CLARK    ON 

(Figs.  81-83),  already  mentioned  as  lying  external  to  the  radial  nerves  at  the  point  where 
they  bend  backwards  over  the  calcareous  ring.  They  are  much  smaller  than  those  figured 
by  Cuenot  ('91)  for  S.  inhaerans  and  differ  from  them  in  having  only  a  single  large 
vesiculated  cell  enclosed  within  them,  instead  of  a  number  of  small  ones.  The  otocysts 
of  S.  vivipara  measure  only  about  60-70/1  in  diameter,  while  the  contained  cell  is  almost  a 
quarter  as  large.  In  no  case  have  I  found  more  than  one  cell  enclosed  in  an  otocyst  of 
this  species.  Hamann  ('84)  suspected  that  they  were  larval  organs  having  no  function 
in  the  adult,  but  Semon  ('87)  has  already  proved  that  idea  erroneous,  as  Hamann  ('89, 
p.  308)  has  since  admitted.  If  any  further  evidence  were  needed,  it  could  be  found  in 
the  increase  of  size  of  the  organs  during  the  development  of  the  animal  (Figs.  81-83)  as 
well  as  in  the  very  obvious  connection  with  the  radial  nerves.  But  I  am  inclined  to  the 
view  that  these  so-called  otocysts  do  not  function  as  hearing  organs  at  all,  but  are  of  use 
to  indicate  the  animal's  position.  Semon  ('87)  was  unable  to  find  any  cilia  in  them,  and 
his  experiments  on  living  Synaptas  brought  him  to  the  conclusion  that  they  were  deaf  to 
sound  waves.  If  the  enclosed  cell  is  vesiculated,  as  it  appears  to  be,  it  must  float  in  the 
fluid  with  which  the  otocyst  is  filled  and  so  presses  on  that  part  of  the  wall  which  is 
uppermost.  Any  change  in  the  position  of  the  animal  would  cause  a  corresponding 
change  in  the  position  of  the  enclosed  cell  and  thus  give  rise  to  a  changed  sensation. 

The  fully  grown  Synapta  mvipara  (Fig.  20)  measures  from  ten  to  fifteen  centi- 
meters in  length  and  from  four  to  nine  millimeters  in  thickness ;  the  size  depending 
largely  on  the  state  of  contraction  of  the  muscles.  In  color  they  vary  from  a  pale 
reddish  brown  to  a  very  dark  greenish  brown  more  or  less  spotted  and  blotched  with 
white.  The  ground  color  is  due  to  the  pigment  in  the  connective  tissue  of  the  body-wall 
and  varies  greatly  with  the  amount  of  that  pigment,  but  the  white  spots  and  blotches  are 
due  to  the  aggregation  of  great  numbers  of  the  miliary  granules,  just  beneath  the 
ectoderm.  The  pigment  is  not  affected  to  any  extent  by  pure  alcohol,  but  corrosive 
sublimate  and  all  acids  destroy  or  greatly  modify  it.  Just  posterior  to  the  calcareous 
ring  and  in  connection  with  it  there  is  a  ring  of  cartilage-like  connective  tissue  (Fig. 
90).  This  structure  was  described  and  figured  by  Theel  ('86),  who  also  figured  the 
anchors  and  plates  from  the  body-wall,  in  his  account  of  S.  picta.  The  anchors  (Fig.  51) 
lie  close  under  the  ectoderm  and  parallel  with  it,  at  right  angles  to  the  main  axis  of  the 
body.  Each  anchor  is  much  curved  or  bowed  inwards,  while  its  arms  or  flukes  are  curved 
outward  so  that  the  points  of  the  arms  are  always  projecting.  The  vertex  is  not  toothed 
but  has  five  or  six  almost  spherical  knobs  on  its  edge.  The  posterior  end  is  broadened 
out  into  several  short,  very  finely-toothed  branches.  Beneath  the  anchors  lie  the  rounded, 
smooth-edged,  somewhat  arched  plates,  which  normally  possess  seven  large  toothed  holes 


SYNAPTA   VIVIPARA.  79 

(Fig.  52)  and  two  large  and  three  small  smooth  holes.  On  the  side  of  the  plate  next 
to  the  anchor  and  near  the  posterior  end  is  an  arched  bow,  which  bears  a  few  teeth  on 
its  anterior  edge.  Increased  growth  of  the  plate  often  increases  the  number  of  holes 
(Fig.  53),  but  as  a  rule  they  are  very  constant.  The  calcareous  rods  which  were  so 
abundant  in  the  tentacles  of  the  young  larvae  are  so  few  that  for  a  long  time  I  was  led 
to  consider  them  entirely  wanting.  The  tentacles  (Fig.  57)  of  the  adults  are  long  and 
slender  with  from  12  to  18  pairs  of  digits,  but  the  number  varies  greatly  with  the  age 
and  size  of  the  animal.  The  glandular  organs  which  characterize  the  young  ten- 
tentacled  stage  seem  to  be  entirely  wanting  now ;  at  any  rate,  I  have  never  found  any 
trace  of  them  in  an  adult. 

A  number  of  interesting  monstrosities  were  found,  chiefly  among  the  older  embryos. 
One  of  these  is  shown  in  Fig.  93,  but  some  of  the  others  were  much  more  complicated, 
consisting  of  three,  four,  and,  in  one  case,  five  young,  which  had  grown  together,  or 
budded  from  each  other  in  various  ways.  Among  adults,  besides  the  rather  common 
addition  of  an  extra  tentacle,  the  only  peculiar  specimen  found  was  one  which  -had  only 
three  radial  muscles  and  nerves  and  only  eleven  tentacles.  There  were  three  tentacles 
in  the  mid-dorsal  interradius  (indicated  by  the  mesentery),  and  four  tentacles  in  each  of 
the  lateral  interradii. 

9.  CONCLUSIONS. 

Probably  no  theory  of  echinoderm  phylogeny  has  attracted  more  attention  or  seems 
more  plausible  than  that  upon  which  Semon  ('88)  determined,  as  the  result  of  his 
studies  on  the  development  of  the  auricularia  larva,  of  Synapta  digitata.  Although  it 
still  finds  supporters  at  the  present  time,  the  investigations  of  Ludwig  ('91)  on  Cucu- 
maria  and  of  Ludwig  and  Barthels  ('91)  on  the  anatomy  of  the  Synaptidae  have  shown 
the  incorrectness  of  Semon's  views,  while  the  observations  of  Bury  ('89  and  '95)  have  cast 
doubt  on  his  interpretation  of  some  of  the  conditions  in  auricularia.  It  is  not  my  inten- 
tion to  enter  here  into  any  discussion  of  the  phylogeny  of  the  echinoderms  but  only  to 
suggest  some  of  the  points  in  the  phylogeny  of  the  holothurians,  upon  which  the  history 
of  Synapta  vivipara  seems  to  throw  some  light,  and  to  indicate  some  of  the  particulars  in 
which  my  studies  have  apparently  offered  support  to  Bury's  ('95)  theory  of  the  ances- 
tral form  of  the  echinoderms. 

There  are  three  possible  opinions  concerning  the  relationship  of  the  Synaptidae  to 
the  other  holothurians  :  first,  Semon's  ('88)  view  that  Synapta  represents  a  primitive 
form,  from  which  the  other  holothurians  have  been  derived;  second,  Cuenot's  ('91)  view 


gO  HUBERT   LYMAN   CLARK   ON 

that  the  Synaptas  represent  a  more  primitive  branch  of  the  echinoderms  than  and  dif- 
ferent from  the  true  holothurians ;  third,  Ludwig's  (*89-'92)  view  that  the  Synaptidae 
are  degenerate,  pedate  holothurians.  Semon  bases  his  opinion  on  the  high  development 
of  the  nervous  system  in  Synapta,  the  absence  of  anything  in  their  manner  of  life  to 
cause  degeneration,  and  the  fact  that  no  organs  appear  in  the  development  of  the  young 
Synapta  which  are  not  present  in  the  adult.  His  own  observations  on  the  nervous  sys- 
tem of  Synaptas  as  well  as  Hamann's  ('83)  and  Cuenot's  ('91)  show  that  there  is  some 
tendency  to  diversity  in  the  nervous  system,  especially  as  regards  sense-organs,  among 
the  Synaptidae,  and  it  also  shows  a  considerable  degree  of  adaptability  to  changing 
conditions.  Moreover,  I  have  found  in  S.  vivipara  that  the  sense-organs  are  highly 
developed,  and  there  are  additional  nerves  to  the  oesophagus,  indicating  modifications 
to  suit  the  mode  of  life.  It  seems  from  these  facts,  that  too  much  stress  must  not 
be  placed  on  the  opinion  that  the  nervous  system  of  Synapta  digitata  is  primitive. 
As  regards  degeneracy  and  the  absence  of  anything  in  the  mode  of  life  to  cause  it  in 
SynaptaSj  it  seems  that  Semon  has  expressed  an  erroneous  opinion  of  the  causes  of 
degeneration.  He  says  that  we  know  of  only  three  reasons  for  its  occurrence,  parasitic, 
fixed,  or  subterranean  life,  and,  since  none  of  these  are  characteristic  of  Synaptas,  they 
cannot  be  degenerate.  Had  he  given  the  matter  more  careful  consideration  he  would 
have  seen  how  untenable  his  position  is.  Certainly  no  one  will  deny  that  the  loss  of  the 
power  of  flight  in  certain  birds,  as  the  famous  New  Zealand  ground  parrot,  is  degenera- 
tion, yet  they  are  neither  parasitic,  fixed,  nor  subterranean.  Any  change  in  the  mode  of 
life,  due  to  a  change  in  environment,  may  result  in  degeneracy.  The  word  has  corne  to 
have  a  bad  significance  so  that  we  think  of  it  as  indicating  that  the  animal  is  on  the 
down-hill  road,  whereas  it  strictly  means  that  the  animal  has  lost  some  organ  or  group  of 
organs  which  its  ancestors  possessed  and  so  has  become  less  complex  than  they.  Such  a 
loss  must  necessarily,  however,  always  be  a  gain  to  the  species  involved,  otherwise  it 
could  never  have  come  about.  Now,  it  is  entirely  conceivable  that  in  certain  conditions 
of  life  on  the  ocean  bottom,  in  shallow  water  near  shore,  the  loss  of  numerous  ambulacral 
appendages  and  the  concentration  of  the  water-vascular  system  in  the  circumoral  ten- 
tacles would  be  a  distinct  advantage  to  the  animal.  Such  has  certainly  been  the  case 
in  Caudina  (Gerould,  '96),  for  instance,  and  it  is  probably  true  of  all  the  Molpadiidae, 
although  in  these  cases  it  may  have  been  brought  about  by  subterranean  life.  But 
Semon  has  by  no  means  proved  his  point  that  the  Synaptas  are  not,  as  a  rule,  subterranean. 
Whatever  may  be  the  condition  at  Naples,  both  on  the  New  England  coast  and  in  Jamaica 
Synapta  inkaerans  and  its  allied  forms  are  found  normally  buried  deep  in  the  sand,  while 
the  large  Synaptas,  like  S.  lappa,  are  found  under  stones,  which  is  practically  a  sub- 


SYNAPTA   VIVIPARA.  81 

terranean  mode  of  life.  The  absence  of  anything,  therefore,  in  their  manner  of  life  to 
cause  degeneration  is  by  no  means  proven  and  will  hardly  stand  as  a  good  test  for  con- 
sidering the  Synaptas  primitive.  The  statement  that  there  is  no  structure  developed  in 
the  young  Synapta  which  does  not  appear  in  the  adult  is  completely  refuted  by  the 
careful  observations  of  Ludwig  and  Barthels  ('91)  on  the  absence  of  radial  water-canals 
in  the  Synaptidae.  Since  all  observers  are  agreed  that  radial  canals  are  developed  in  the 
embryo  of  S.  diyitata,  it  is  clear  that  we  have  here  a  most  important  structure  lost  in  the 
adult.  For  these  reasons,  it  seems  to  me  that  Semon's  view  is  no  longer  tenable. 
Cuenot's  ('91)  view  is  based  chiefly  on  the  important  differences  in  the  embryology  of  the 
Synaptidae  and  that  of  other  holothurians,  but  it  seems  to  me  that  he  does  not  take  suffi- 
cient account  of  the  important  evidences  of  degeneration  in  the  Synaptas. 

Ludwig's  ('89-'92)  view  appears  to  be  the  one  best  supported  by  the  facts,  and  the 
anatomy  and  embryology  of  S.  vivipara  offer  no  little  confirmatory  evidence.  If  we 
compare  its  ten-tentacled  stage  with  the  hypothetical  ancestor  which  Ludwig  describes 
for  the  Synaptidae,  the  resemblance  is  extraordinary,  almost  the  only  important  differ- 
ence being  that  the  genital  gland  in  S.  vimpara  is  not  equally  developed  on  each  side. 
In  fact,  the  ten-tentacled  stage  of  Synapta  mvipara  represents  an  actual  step  in  the 
development  of  the  Synaptidae  from  Ludwig's  hypothetical  ancestor.  The  Jamaican 
species  is  beyond  doubt  a  highly  modified  form,  and,  though  in  some  respects  more  highly 
organized  than  other  Synaptas,  in  certain  particulars,  degeneration  has  gone  further. 
Differing  from  other  holothurians  in  its  manner  of  life  and  its  mode  of  reproduction,  it 
has  undergone  various  modifications  to  fit  it  for  the  changed  conditions.  Living  in  sea- 
weed near  the  surface  of  the  water,  it  has  developed  pigment  in  its  skin  to  a  marked 
degree,  and  at  the  same  time  has  acquired  additional  sense-organs  in  the  eyes  at  the  base 
of  the  tentacles,  and  an  increased  innervation  of  the  oral  disc.  In  conformity  to  its 
changed  mode  of  reproduction,  important  changes  have  taken  place  in  the  structure  of 
the  genital  gland,  openings  have  appeared  in  the  walls  of  the  rectum  to  connect  the  body- 
cavity  with  the  exterior,  while  the  stone-canal  has  retained  or  has  acquired  secondarily 
its  original  connection  with  the  exterior.  During  the  progress  of  these  specializations, 
the  same  causes  have  led  to  degeneration  in  other  particulars.  The  changed  mode  of 
reproduction  has  modified  the  genital  duct,  so  that  its  lumen  is  no  longer  open  to  the 
ova,  and  it  no  longer  has  an  obvious  opening  to  the  exterior.  The  changed  mode  of 
life  has  caused  a  greater  concentration  of  the  water- vascular  system  around  the  mouth 
and  a  consequent  further  degeneration  of  the  radial  canals,  so  that  they  no  longer  appear 
as  such  even  in  the  embryology,  but  tentacles  develop  directly  from  the  secondary  out- 
growths of  the  hydrocoel.  And,  furthermore,  the  mid-ventral  outgrowth  has  degenerated 


82  HUBERT  LYMAN   CLARK   ON 

a  step  further  and  normally  never  develops  at  all,  but  disappears  altogether,  which  is 
especially  interesting  as  the  mid-ventral  radius  is  the  first  to  develop  its  nerve  and  oto- 
cysts,  and  so  seems  to  be  the  leader  in  modifications.  The  manner  of  life  has  also  caused 
a  modification  of  the  tentacles  in  a  way  which  we  may  consider  as  a  degeneration  from 
other  Synaptas.  Semon  ('87)  describes  calcareous  rods  in  the  tentacles  of  all  the  Medi- 
terranean Synaptas,  and  these  appear  abundantly,  as  we  have  seen,  in  the  young  stages  of 
S.  mvipara,  but  in  the  adult  they  seem  to  have  almost  entirely  disappeared,  the  tentacles 
and  digits  being  very  delicate  and  flexible  and  containing  no  calcareous  deposits,  except 
some  miliary  granules.  The  changes  in  the  larva  due  to  the  retention  and  development 
of  the  ova  in  the  body-cavity  of  the  mother,  such  as  closing  of  the  blastopore  and  absence 
of  any  true  metamorphosis,  must  also  be  taken  into  consideration.  For  these  reasons,  we 
must  consider  8.  mvipara  as  a  highly  specialized  Synapta,  but  in  its  water-vascular  system 
it  has  degenerated  a  step  further  than  S.  digitata,  although  it  is  neither  "parasitic,  fixed, 
nor  subterranean  "  in  its  manner  of  life. 

It  is  very  clear  from  the  examination  of  the  literature  on  the  subject  that  the  study 
of  any  one  form  or  class  of  echinoderms  is  entirely  insufficient  to  fit  one  to  determine  on 
a  theory  of  the  phylogeny  of  the  group.  Notable  examples  of  this  may  be  seen  in  the 
speculations  of  Semon  ('88),  Butschli  ('92),  and  MacBride  ('96).  The  facts  added  to 
our  knowledge  of  echinoderm  embryology  by  all  these  writers  are  of  real  value,  but  their 
hypotheses  are  for  the  most  part  of  little  importance.  The  same  may  be  said  of  any 
attempt  to  determine  the  entire  course  of  echinoderm  evolution  by  the  study  of  palaeon- 
tology alone,  a  notable  example  of  which  has  recently  appeared  by  no  less  an  authority 
than  Haeckel  ('96).  The  only  author  who  has  carried  on  original  investigations  on  all 
the  classes  of  echinoderms  and  has  formulated  his  views  on  the  phylogeny  of  the  group  is 
Bury  ('80  and  '95),  and  I  cannot  conclude  this  paper  without  calling  attention  to  the 
support  which  my  observations  give  to  him,  on  the  questions  involved  in  the  development 
of  the  Synaptidae.  Regarding  all  the  points  on  which  he  lays  particular  stress,  I  have 
confirmed  his  work  completely  or  in  part.  The  adradial  position  of  the  water-tube,  the 
rudimentary  left  anterior  enterocoel,  and  the  growth  of  the  left  body-cavity  around  the 
oesophagus  are  all  very  clearly  marked  in  the  development  of  Synapta  mmpara.  The 
only  point  on  which  I  could  not  entirely  confirm  his  views  was  on  the  formation  of  the 
mesentery  of  the  stone-canal  from  the  left  coelom  entirely,  and  on  this  point  what  evi- 
dence I  did  obtain  indicates  the  correctness  of  his  position. 


SYNAPTA   VIVIPARA.  33 


10.   LITERATURE. 

[Articles  marked  *  contain  references  to  Synapta  vivipara.] 
Agassiz,  A. 

'64.   On  the  embryology  of  echinoderms.     Mem.  Amer.  acad.  arts  and  sci.,  Vol.  9,  p.  1-30. 
Baur,  A. 

'04.  Beitrage  zur  naturgeschichte  der  Synapta  digitata.    Nova  acta  acad.  Leop.-Carol.,  Bd.  31,  119  pp.,  8  taf. 
*Bronn,  II.  G. 

'60.   Die  klassen  und  ordnungen  der  strahlenthiere   (Actinozoa).    Bronn's  Klassen  und  ordnungen  des  thierreichs- 

Leipzig  und  Heidelberg,  434  pp.,  48  taf. 
Btttschli,  O. 

'92.   Versuch  der  ableitung  des  echinodenns  aus  einer  bilateralen  urform.    Zeitschr.  f.  wiss.  zool.,  Bd.  53,  suppl.,  p.  136- 

160,  taf.  9. 
Bury,  H. 

'89.   Studies  in  the  embryology  of  echinoderms.    Quart,  journ.  micros,  sci.,  Vol.  29,  p.  409-449,  pis.  37-39. 
Bury,  H. 

'95.   The  metamorphosis  of  echinoderms.     Quart,  journ.  micros,  sci.,  Vol.  38,  p.  45-135,  pis.  3-9. 
*  Clark,  H.  L. 

'96.  The  viviparous  Synapta  of  the  West  Indies.    Zool.  anz.,  Bd.  19,  p.  398-400. 
Cut-not,  L. 

'91.  Etudes  morphologiques  sur  les  echinodermes.     Arch,  de  biol.,  Tom.  11,  p.  313-680,  pis.  24-31.     See  also  Arch.  zool. 

exper.,  (2),  Tom.  9,  Notes  et  Rev.,  p.  8-16. 
Danielssen,  1).  C.,  and  Koren,  J. 

'82.   Holothurioidea.     Norwegian  North  Atlantic  "Expedition,  1876-78.    6.  Zoology.    Christiania,  94  pp.,  13  pis.,  1  map. 
Gerould,  J.  H. 

'96.  The  anatomy  and  histology  of  Caudina  arenata  Gould.    Proc.  Bost.  soc.  nat.  hist.,  Vol.  27,  p.  7-74,  8  pis.    Also 

Bull.  mus.  comp.  zool.,  Vol.  29,  p.  121-190,  8  pis. 
Goette,  A. 

'80.   Bemerkungen  zur  entwickelungsgeschichte  der  echinodermen.     Zool.  anz.,  Jahrg.  3,  p.  324-326. 
Haeckel,  E. 

'00.  Die  amphorideen  und  cystoideen.    Beitrage  zur  morphologic  und  phylogenie  der  echinodermen.    Festschrift  zum 

siebenzigsten  geburtstage  von  Carl  Gegenbaur,  Bd.  1,  p.  1-179,  4  taf. 
Hamann,  O. 

'83.   Beitrage  zur  histologie  der  echinodermen.     1.   Die  holothurien  (Pedata)   und  das  nervensystem  der  asteriden. 

Zeitschr.  f.  wiss.  zool.,  Bd.  39,  p.  145-190,  taf.  10-12. 
Hamann,  0. 

'84.  Beitrage  zur  histologie  der  echinodermen.     Heft  1,  Die  holothurien.    Jena,  100  pp.,  6  taf. 
Hamann,  O. 

'89.   Anatomie  der  ophiuren  und  crinoiden.    Jena,  zeitschr.,  Bd.  23,  p.  233-388,  taf.  12-23. 
Hiirouard,  E. 

'89.   Recherches  sur  les  holothuries  des  cotes  de  France.     Arch.  zool.  expfer.,  (2),  Tom.  7,  p.  535-704,  pis.  25-32. 
Jourdan,  E. 

'83.  Recherches  sur  I'histologie  des  holothuries.    Ann.  mus.  hist.  nat.  Marseille.  Zool.,  Tom.  1,  M6m.  6,  64  pp.,  pis.  1-6. 
Kowalevsky,  A. 

'67.   Beitrage  zur  entwickelungsgesehichte  der  holothurien.     M6m.  acad.  sci.  St.  Pfetersbourg  (7),  Tom.  11,  8  pp.,  1  pi. 
*  Lampert,  K. 

'85.   Die  seewalzen.    Reisen  in  Archipel  der  Philippinen.    Von  Dr.  C.  Semper,  Bd.  4,  Heft  3,  312  pp.,  pi.  1.   Wiesbaden. 
*Ludwig,  H. 

'81.  Ueber  eine  lebendiggeb&rende  synaptide  und  zwei  andere  neue  holothurienarten  der  brazilianischen  ktiste.     Arch. 

de  biol.,  Tom.  2,  p.  41-58,  pi.  3. 
*Ludwig,  H. 

'86.   Die  von  G.  Chierchia  auf  der  fahrt  der  Kgl.  Ital.  Corvette  "  Vettor  Pisani"  gesammelten  holothurien.    Zool.  jahr- 
biicher,  Bd.  2,  p.  1-36,  taf.  1-2. 


84  HUBERT   LYMAN  CLARK   ON 

*  Ludwig,  H. 

'89-92.     Echinodermen.     Bronn's  Klassen  und   ordnungen   des  thierreichs,  Bd.  2,  Abth.  3.     Lieferungen  1-16  (Holo- 

thurien). 
Ludwig,  H. 

'91.   Zur  eutwickehmgsgeschichte  der  holothurien.     Sitzungsber.  k.  Preuss.  akad.  wiss.,  No.  10,  p.  179-192,  No.  32,  p.  603- 
612.     Transl.  in  Ann.  and  mag.  nat.  hist.,  (6),  Vol.  8,  p.  413-427. 

*  Ludwig,  H.,  und  Barthels,  P. 

'91.   Zur  anatomie  der  synaptiden.     Zool.  anz.,  Jahrg.  14,  p.  117-119. 
MacBride,  E.  W. 

'96.   The  development  of  Asterina  gibbosa.     Quart,  journ.  micros,  sci.,  Vol.  38,  p.  339-411,  pis.  18-29. 
Metschnikoff,  E. 

'69.   Studien  liber  die  entwickelung  der  echinodermen  und  nemertinen.     M£m.  acad.  sci.  St.  Petersbourg,  (7),  Tom.  14, 

73  pp.,  12  pis. 
Mortensen,  T. 

"94.   Zur  anatomie  und  entwicklnng  der  Cucumaria  glacialis  (Ljungman).     Zeitschr.  f.  wiss.  zool.,   Bd.  57,  p.  704-732, 

taf.  31-32. 
MUller,  J. 

'50.   Anatomische  studien  iiber  echinodermen.     Miiller's  archiv,  Jahrg.  1850,  p.  117-165. 
Miiller,  J. 

'52.   Ueber  Synapta  digitata  und  iiber  die  erzeugung  von  schnecken  in  holothurien.     Berlin,  1852,  36  pp.,  10  taf. 
Orated,  A.  S. 

'49.    [Slaegt  of  Synapta  gruppen.]    Videns.  medd.  nat.  for.  Kjobenhavn,  Aarene  1849  og  1860,  p.  vii.   Translated  in 

Ludwig  ('81),  p.  48. 
Quatrefages,  A.  de. 

'42.   Memoire  sur  la  synapte  de  Duvernoy  (S.  duvernaea  A  de  Q).     Ann.  sci.  nat ,  Zool.,  (2),  Tom.  17,  p.  19-93,  pi.  2-6. 
Selenka,  E. 

'67.   Beitrage  zur  anatomie  und  systematik  der  holothurien.    Zeitschr.  f.  wiss.  zool.,  Bd.  17,  p.  291-372,  taf.  17-20 
Selenka,  E. 

'76.  Zur  entwickelung  der  holothurien  (Holothuria  tubulosa  u.  Cucumaria  doliolum).    Zeitschr.  f.  wiss.  zool.,  Bd.  27, 

p.  156-178. 
gelenka,  E. 

'83.   Die  keirnblatter  der  echinodermen.     Studien  ueber  entwickelungsgeschichte  der  thiere.     Heft  2,  p.  28-61,  6  taf. 

Wiesbaden. 
Senion,  R. 

'83.  Das  nervensystem  der  holothurien.    Jena,  zeitschr.,  Bd.  16,  p.  1-23,  taf.  1-2. 
Semon,  E. 

'87.   Beitrage  zur  naturgeschichte  der  synaptiden  des  Mittelmeeres.    Mittheil.  zool.  station  Neapel,  Bd.  7,  p  272-300; 

p.  401-422,  taf.  9,  10,  u.  15. 
Semon,  R. 

'88.   Die  entwickelung  der  Synapta  digitata  und  die  stammesgeschichte  der  echinodermen.    Jena,  zeitschr.,  Bd.  22,  p.  175- 

309,  taf.  6-12. 
Semon,  R. 

'89.   Die  homologien  innerhalb  des  echinodermenstammes.     Morph.  jahrb.,  Bd.  15,  p.  253-307. 
Semper,  C. 

'68.   Reisen  im  Archipel  der  Philippine!!.     2.   Wissenschaftliche  resultate.    1.   Holothurien,  288  pp.,  40  taf.     Leipzig. 
Theel,  H. 

'82.  Report  on  the  Holothuroidea.    Pt.  1.  "  Challenger  "  Reports.     Zoology,  Vol.  14,  Part  13,  London. 
Th6el,  H. 

'86.  Report  on  the  Holothuroidea.     Pt.  2.    "  Challenger  "  Reports.    Zoology,  Vol.  14,  Part  39.    London. 
Thomson,  W. 

'62.   On  the  development  of  Synapta  inhaerens,  O.  F.  MUller  (sp.).     Quart,  journ.  micros,  sci.,  Vol.  2,  p.  131-146. 


SYNAPTA    VIVIPARA. 


85 


11.   EXPLANATION  OF  PLATES. 

[AH  figures  except  20  and  23  were  drawn  with  the  aid  of  a  camera  lucida.] 
ABBREVIATIONS    USED. 


A.  —  atrium. 

AE.  —  anterior  euterocoel. 
AN.  —  anus. 
AO.  —  atrial  opening. 
AT.  — accessory  tentacle. 
BC —  body-cavity. 
BL.  —  blastopore. 
BV.  —  blood-vessel. 
CF.  —  ciliated  funnels. 

CH.  —  cireumoesophageal  ring  of  blood-system. 
CM.  — circular  muscles  of  body-wall. 
CR.  —  calcareous  ring. 
CT.  —  connective  tissue. 
Car.R.  —  cartilaginous  ring. 

Cir.S.  —  circular  sinus  formed  from  the  anterior-prolon- 
gation of  the  left  coelom. 
DV.  —  dorsal  vein  of  blood-system. 
E.  —  enterocoel. 
Ect.  —  ectoderm. 
Epi.  —  epithelium. 
Ey.  —  eyes. 
GD.  —  genital  duct. 
GG.  —  genital  gland. 
H. — hydrocoel. 
I.  —  intestine. 
LC.  —  left  coeloin. 


LGO.  —  larval  glandular  organ. 

LM.  —  longitudinal  muscles. 

M.  —  mouth. 

Mes.  —  mesenchyme. 

MD.  —  madrepore. 

MY.  —  mesentery. 

NR.  —  circumoral  nerve-ring. 

O.  —  otocysts. 

OE.  —  oesophagus. 

OEN.  —  nerve-band  to  mouth  and  oesophagus. 

PT.  —  primary  tentacle. 

PV. —  Polian  vesicle. 

R.  —  rectum. 

RC.  —  right  coelom. 

RN.  —  radial  nerve. 

SC.  —  stone-canal. 

SO.  —  secondary  outgrowth  of  the  hydrocoel. 

SP.  —  sense-papilla. 

T.  —  tentacle. 

TC.  —  canal  of  tentacle. 

TN.  —  tentacle-nerve. 

TV.  —  blood-vessel  on  inner  side  of  tentacular  canal. 

V.  —  valves. 

WP.  — .  water-pore. 

WR.  —  water-ring. 


86  HUBERT  LYMAN    CLARK   ON 


PLATE   11. 

Fig.  1.  Mature  ovmn.    225x. 

Fig.  2.  Two-cell  stage  of  segmenting  egg.     225x. 

Fig.  3.  Four-cell  stage.    225x. 

Fig.  4.  Eight-cell  stage.     225x. 

Fig.  5.  Sixteen-cell  stage.    225x. 

Fig.  6.  Thirty-two-cell  stage,  seen  from  the  side.    225x. 

Fig.  7.  Thirty-two-cell  stage,  seen  from  one  of  the  poles.    225x. 

Fig.  8.  Blastula,  seen  from  the  side.    225x. 

Fig.  9.  Gastrula,  seen  from  the  side.    225x. 

Fig.  10.  Older  gastrula,  seen  from  left-hand  side,  to  show  formation  of  the  water-pore. 

Fig.  11.  Still  older  stage,  seen  from  left-hand  side,  to  show  the  drawing  away  of  the  archenteron  from  the  water- 
pore.  225x. 

Fig.  12.  Slightly  older  stage,  seen  from  left  side,  to  show  the  formation  of  the  mouth.    225x. 

Fig.  13.  Older  stage,  seen  from  in  front  (ventrally),  to  show  the  formation  of  the  enterocoel.     225x. 

Fig.  14.  Older  stage,  seen  from  in  front  (veutrally),  to  show  the  formation  of  the  coelomic  vesicles.    225x. 

Fig.  15.  Older  stage,  seen  from  in  front  (ventrally),  to  show  the  five  primary  outgrowths  of  the  hydrocoel.     225x. 

Fig.  16.  Older  stage,  seen  from  left  side,  to  show  the  hydrocoel,  body-cavities,  and  atrium.     225x. 

Fig.  17.  Very  young  pentactula,  from  right  side,  to  show  the  nerves  and  sense-organs.    225x. 

Fig.  18.  Older  pentactula,  seen  from  dorsal  surface,  to  show  the  radial  nerves,  calcareous  ring,  and  rudiment  of  accessory 
tentacle.  130x. 

Fig.  19.  Ten-tentacled  young,  seen  from  right  side,  to  show  genital  gland  and  arrangement  of  organs.  Twelfth  tentacle 
just  developing.  22x. 

Fig.  20.  Adult  Synapta  vivipara,  seen  from  ventral  surface.    Nat.  size. 


PLATE   12. 

Fig.  21.    Vertical  section  of  gastrula,  to  show  the  thickened  ectoderm  at  apical  pole.     225x. 

Fig.  22.    Transverse  section  of  Fig.  13,  at  the  line  A.  B.,  to  show  the  thickened  ventral  ectoderm.     225x. 

Fig.  23.     Schematic  outline  of  hydrocoel,  to  show  the  position  of  water-canal. 

Fig.  24.  Transverse  sections  of  hydrocoels  of  three  embryos,  to  show  formation  of  anterior  enterocoel.  a,  youngest 
stage;  b,  somewhat  older;  c,  oldest  stage.  225x.  In  c,  only  a  very  small  part  of  the  hydrocoel  is  shown. 

Fig.  25.  Transverse  section  of  larva,  somewhat  older  than  Fig.  16,  to  show  closure  of  hydrocoel  ring,  without  the  forma- 
tion of  a  Polian  vesicle.  225x.  The  section  is  very  oblique,  and  takes  in  a  part  of  the  floor  of  the  atrium  (A),  and  only  a 
portion  of  the  hydrocoel. 

Fig.  26.    Transverse  section  of  larva  like  Fig.  16,  to  show  anterior  prolongation  of  the  left  coeloin.    225x. 

Fig.  27.    Transverse  section  of  same  larva,  somewhat  higher  up,  to  show  the  left  prolongations  of  the  left  coelom.     225x. 

Fig.  28.  Longitudinal  section  of  larva  like  Fig.  16,  very  badly  preserved,  to  show  the  anterior  prolongation  of  the  left 
coelom,  ant.  1.  c.  225x. 

Fig.  29.    Posterior  end  of  an  adult,  to  show  the  rupture  of  the  body-wall,  caused  by  birth  of  the  young.    35x. 

Fig.  30.  Transverse  section  of  the  posterior  end  of  an  adult,  to  show  one  of  the  openings  from  the  rectum  into  the  body- 
cavity.  65x. 

Fig.  81.    One  of  these  openings  more  highly  magnified.    500x. 

Fig.  32.    Part  of  a  transverse  section  of  a  young  pentactula,  to  show  the  origin  of  the  genital  gland.     500x. 

Fig.  33.    Similar  section  of  an  older  larva,  to  show  the  increased  development  of  the  genital  gland.    500x. 

Fig.  34.  Similar  section  of  a  still  older  larva,  to  show  first  appearance  of  lumen  and  covering  epithelium  of  the  genital 
gland.  I,  beginnings  of  lumen;  epi,  outer  epithelium  of  gland,  formed  secondarily  from  right  side  of  mesentery.  600x. 

Fig.  35.  Similar  section  of  a  young  twelve-tentacled  larva,  to  show  the  genital  gland  well  developed  on  the  right-hand 
side  of  mesentery  and  confined  to  that  side.  500x. 

Fig.  36.  Part  of  a  similar  section  of  an  adult,  to  show  the  formation  of  the  lumen  of  the  genital  duct  from  the  lumina  of 
the  glands.  200x. 


SYNAPTA   VIVIPARA.  87 

Fig.  37.  Longitudinal  section  of  a  part  of  genital  duct,  to  show  its  structure  and  position  in  the  mesentery.  I,  lumen  of 
duct;  g.  e.,  germinal  epithelium;  m. e.,  epithelium  of  mesentery.  950x. 

Fig.  38.  Transverse  section  of  dorsal  body-wall  of  an  adult,  to  show  the  termination  of  the  genital  duct,  sp,  sper- 
matozoa. 225x. 

Fig.  39.  Longitudinal  section  of  a  small  part  of  genital  gland,  to  show  the  position  of  ovum,  pressing  against  the  epi- 
thelium of  the  gland,  epi,  epithelium  of  gland ;  ov,  ovum.  500x. 

Fig.  40.     Transverse  section  of  genital  gland,  to  show  its  structure  and  the  position  of  the  ova  (ov).    500x. 


PLATE   13. 

Fig.  41.  Transverse  section  of  the  ectoderm  of  a  pentactula,  to  show  the  invagination  whiclrforms  the  larval  glandular 
organ.  500x. 

Fig.  42.    Transverse  section  of  the  same  organ,  fully  grown,  from  the  body-wall  of  a  ten-tentacled  larva.    500x. 

Fig.  43.  Longitudinal  section  of  one  of  these  organs,  to  show  the  peripheral  position  of  the  nuclei,  and  the  lumen  at  the 
center.  600x. 

Fig.  44.     Interradial  plate  from  the  calcareous  ring  of  an  adult.     35x. 

Fig.  45.     Radial  plate  from  the  same  ring.     35x. 

Fig.  46.  The  development  of  these  calcareous  plates,  a,  youngest  stage  ;  b-g,  successively  older  stages ;  n,  older  stage 
of  an  interradial  plate ;  i,  older  stage  of  a  radial  plate.  225x. 

Fig.  47.  Part  of  the  calcareous  ring  of  an  old  ten-tentacled  larva,  to  show  the  growth  of  the  radial  plate  (r.p.)  of  the 
calcareous  ring,  in  support  of  the  developing  eleventh  tentacle,  ip,  interradial  plates.  65x. 

Fig.  48.    Calcareous  rods  from  the  tentacles  of  a  ten-tentacled  larva.    225x. 

Fig.  49.     Calcareous  rods  from  around  madrepore.     225x. 

Fig.  50.     Miliary  granules  from  the  skin  of  an  adult.     225x. 

Fig.  61.     Anchor  from  the  skin  of  an  adult.     120x. 

Fig.  52.     Normal  anchor-plate  from  an  adult.     120x. 

Fig.  53.     Abnormal  anchor-plate  from  an  adult.     120x. 

Fig.  54.     Longitudinal  section  of  the  tentacle  of  an  old  pentactula,  to  show  the  thickened  ectoderm  at  the  tip.    225x. 

Fig.  55.  Cross-section  of  a  young  tentacle,  to  show  the  entire  separation  of  the  canals  of  the  digits  from  the  central 
canal  of  the  tentacle.  225x. 

Fig.  56.  Longitudinal  section  of  part  of  a  young  tentacle,  to  show  the  origin  of  the  digits,  as  outgrowths  of  the  tentacle- 
canal.  226x. 

Fig.  57.    Tentacle  of  an  adult.    22x. 

Fig.  58.  Transverse  section  of  a  mesentery  and  intestine  of  a  pentactula,  to  show  the  origin  of  the  haemal  system  from 
the  right  lamina  of  the  mesentery.  500x. 

Fig.  69.     Loop  of  intestine  in  a  small  adult,  to  show  formation  of  the  transverse  vessels  of  the  haemal  system.     35x. 

Fig.  60.     The  same  from  an  older  specimen.     65x. 

Fig.  61.  Cross-section  of  the  dorsal  mesentery  of  a  larva  one  mm.  long,  to  show  the  beginning  of  the  ciliated  funnels. 
575x. 

Fig.  62.  The  development  of  the  ciliated  funnels,  a,  the  first  large  cells  seen  from  above  looking  down  on  the  surface 
of  mesentery  ;  6,  increased  number  of  cells ;  c,  slightly  older  stage  seen  from  the  side  ;  d,  older  stage,  surface  view  ;  e,  older 
stage,  seen  partly  from  the  side ;  /,  older  stage  seen  from  the  side.  575x.. 

Fig.  63.     Longitudinal  section  of  a  funnel,  to  show  its  structure.     950x. 

Fig.  64.     Ciliated  funnel  of  an  adult,  seen  from  in  front.     575x. 

Fig.  65.    Ciliated  funnel  of  an  adult,  seen  from  above.    225x. 

Fig.  66.     Surface  view  of  stone-canal  of  an  adult,  to  show  its  course  from  water-ring  to  the  exterior.    35x. 


PLATE   14. 

Fig.  67.  Cross-section  of  stone-canal  and  genital  duct  of  an  adult  before  they  enter  the  body-wall.    225x. 

Fig.  68.  Cross-section  of  stone-canal  and  genital  duct  of  same  adult  in  the  body-wall.    225x. 

Fig.  69.  Cross-section  of  same  stone-canal  as  it  approaches  the  body-wall.    225x. 

Fig.  70.  Cross-section  of  the  same,  where  it  opens  to  the  exterior.    225x. 


88  HUBERT  LYMAN    CLARK  ON    SYNAPTA  VIVIPARA. 

Fig.  71.  Cross-section  of  the  terminus  of  a  stone-canal  with  two  openings,  to  show  the  uppermost  opening.     500x. 

Fig.  72.  Cross-section  from  the  same  series,  two  sections  lower  down.     500x. 

Fig.  73.  Cross-section  of  the  same  series,  two  sections  further  down,  to  show  the  second  opening.     500x. 

Fig.  74.  Cross-section  of  a  stone-canal  of  an  adult,  taken  through  the  madrepore  to  show  the  openings.     225x. 

Fig.  75.  Longitudinal  section  of  nerve-ring  of  an  adult  (transverse  section  of  the  animal),  to  show  the  nerve-band  to  the 
mouth  and  oesophagus  in  cross-section.  200x. 

Fig.  76.  Transverse  section  of  nerve-ring  (longitudinal  section  of  animal),  to  show  the  same  nerve  in  sagittal  section. 
200x. 

Fig.  77.  Longitudinal  section  of  nerve-ring  and  transverse  section  of  tentacle-nerve,  to  show  the  position  of  the  eyes. 
120  x. 

Fig.  78.  Cross-section  of  nerve-ring  and  one  of  the  eyes,  to  show  their  relative  position.     225x. 

Fig.  79.  Cross-section  of  the  tip  of  one  of  the  eyes,  to  show  the  vacuolated  structure  of  the  mesenchymatous  covering  as 
well  as  of  the  polygonal  neive-cells.  950x. 

Fig.  80.  Sagittal  section  of  one  of  the  eyes,  to  show  the  shape  and  structure  of  the  nerve-cells.    950x. 

Fig.  81.  Cross-section  of  an  otocyst  and  its  single  vesiculated  cell  from  a  pentactula.     500x. 

Fig.  82.  Cross-section  of  the  otocysts  and  a  part  of  the  radial  nerve  from  a  twelve-tentacled  young  five  mm.  long. 
500x. 

Fig.  83.  Cross-section  of  an  otocyst  of  an  adult,  showing  the  single  vesiculated  cell-'within.     500x. 

Fig.  84.  Cross-section  of  a  sense-papilla  in  the  ectoderm  of  an  adult,  to  show  its  structure  and  nerve  connections,  ga, 

ganglion  ;  n,  nerve  running  to  radial  nerve.  500x. 


PLATE   15. 

Fig.  85.  Transverse  section  of  an  old  pentactula,  to  show  the  formation  of  the  five  accessory  tentacles  and  the  oto- 
cysts ;  the  secondary  outgrowths  of  the  left  dorsal  and  mid-ventral  radii  have  not  developed  at  all  as  yet.  The  section  is 
slightly  oblique,  more  posterior  on  the  left  than  on  the  right.  225x. 

Fig.  86.  Transverse  section  of  an  old  ten-tentacled  stage,  to  show  position  of  the  eleventh  and  twelfth  tentacles.  The 
section  is  somewhat  oblique,  more  posterior  on  the  right  than  on  the  left.  120x. 

Fig.  87.  Longitudinal  section  of  the  anterior  end  of  a  pentactula  ;  on  the  left  through  a  radius  ;  on  the  right  a  little  to 
one  side  of  a  radius.  /,  indicates  the  place  of  fusion  between  the  oesophagus'  and  roof  of  the  atrium.  225x. 

Fig.  88.  Another  section  of  the  same  series;  on  the  right,  through  an  interradius,  just  touching  on  the  side  of  the 
Polian  vesicle ;  on  the  left  a  little  to  one  side  of  an  interradius.  /,  indicates  the  fusion  of  the  oesophagus  with  the  roof  of 
the  atrium.  225x. 

Fig.  89.    Longitudinal  section  of  a  ten-tentacled  young,  to  show  position  of  the  various  organs.    65x. 

Fig.  90.    Longitudinal  section  of  an  adult  through  one  of  the  tentacles,  to  show  the  position  of  the  various  organs.    35x. 

Fig.  91.     Longitudinal  section  of  an  adult  through  a  radius,  to  show  position  of  the  organs.     35x. 

Fig.  92.  Adult  laid  open  in  right  dorsal  radius,  to  show  position  of  the  organs  and  especially  the  blood-vessels  on 
digestive  tract.  Nat.  size. 

Fig.  93.    Larval  monstrosity,  a  double  embryo.     225x.  , 

Fig.  94.    Cross-section  of  longitudinal  muscle  of  adult  at  the  point  of  juncture  with  the  calcareous- ring.     65x. 

Fig.  95.    Cross-section  of  same  muscle,  lower  down.    65x. 

Fig.  96.    Cross-section  of  same  muscle,  lower  down.    65x. 

Fig.  97.    Cross-section  of  same  muscle,  lower  down.    66x. 

Fig.  98.    Cross-section  of  longitudinal  muscle  of  a  smaller  adult,  somewhat  anterior  to  the  middle  of  the  body.     200x. 

Fig.  99.    Cross-section  of  a  longitudinal  muscle  in  the  middle  of  the  body.     80x. 

Fig.  100.  Cross-section  of  a  longitudinal  muscle  at  extreme  posterior  end'of  the  body.    200x. 

Printed,  January,  1898. 


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